Are the Old Testament Saints still in heaven? If God comes back for the second time for us, are we going to meet this people, or are they no more existing?

The Bible is very clear that the Old Testament saints still exist today and that we will see and fellowship with them in Heaven.

The first and foremost evidence we have of this truth is the immortality of the human soul. Man being created in the image of God (Genesis 1:26And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth.
See All...) is a Trichotomy (a three part being) as the Lord is (1 John 5:6-8 [6] This is he that came by water and blood, even Jesus Christ; not by water only, but by water and blood. And it is the Spirit that beareth witness, because the Spirit is truth. [7] For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. [8] And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one.
See All...) and thus has a spirit, soul and body (1 Thessalonians 5:23And the very God of peace sanctify you wholly; and I pray God your whole spirit and soul and body be preserved blameless unto the coming of our Lord Jesus Christ.
See All...).

At death the soul departs the body (Genesis 35:18And it came to pass, as her soul was in departing, (for she died) that she called his name Benoni: but his father called him Benjamin.
See All..., 1 Kings 17:22And the LORD heard the voice of Elijah; and the soul of the child came into him again, and he revived.
See All...) while the body goes to the grave and the spirit returns to God (Ecclesiastes 2:17Therefore I hated life; because the work that is wrought under the sun is grievous unto me: for all is vanity and vexation of spirit.
See All...). The righteous soul continues to exist in Heaven (Paradise for those believers who died before the crucifixion which we will look at later) or Hell for the unbelieving. While it is true that man shares the same “breath of life” with all of God’s animal creation (Genesis 6:17And, behold, I, even I, do bring a flood of waters upon the earth, to destroy all flesh, wherein is the breath of life, from under heaven; and every thing that is in the earth shall die.
See All..., 7:15
See All...) only man was said to have become “a living soul” (Genesis 2:7And the LORD God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul.
See All..., 1 Corinthians 15:45And so it is written, The first man Adam was made a living soul; the last Adam was made a quickening spirit.
See All...) and thus he alone possess the capability of an eternal existence. This truth encompasses every person ever born from the time of Adam’s creation to our present day.

The age in which one lives has no bearing on the immortality of their soul. As was stated previously, the main difference between the death of the saints before the crucifixion and resurrection of the Lord was that they were not granted immediate access to Heaven as we believers are now. Instead, they were gathered to a place of comfort and rest called Paradise which was located in Abraham’s Bosom in the heart of the Earth. There they would abide until the atonement of their souls and the payment for their sins had been accomplished by the Lord Jesus Christ’s sacrifice on the Cross (Hebrews 10). After His resurrection they were loosed from this “holding area” (Ephesians 4:8-10 [8] Wherefore he saith, When he ascended up on high, he led captivity captive, and gave gifts unto men. [9] (Now that he ascended, what is it but that he also descended first into the lower parts of the earth? [10] He that descended is the same also that ascended up far above all heavens, that he might fill all things.)
See All...) and were then able to enter into Heaven where they live to this day waiting for the resurrection of their glorified bodies. We get a glimpse of this dwelling of the deceased in Luke Chapter 16 with Lazarus and the Rich Man and to a lesser extent again with Samuel in 1 Samuel Chapter 28. In both cases these men are said to be dead but yet they continue to live and retain all of their mental and physical capacities that they possessed while alive here on Earth. Obviously then death in the Bible does not mean complete annihilation of a person’s being but rather the transformation from this earthly, physical existence to that of a spiritual realm. That man would cease to exist at his death or drift off into a state of unconsciousness is a false teaching propagated by various cults and is easily refuted by Scripture (Mark 9:44-48 [44] Where their worm dieth not, and the fire is not quenched. [45] And if thy foot offend thee, cut it off: it is better for thee to enter halt into life, than having two feet to be cast into hell, into the fire that never shall be quenched: [46] Where their worm dieth not, and the fire is not quenched. [47] And if thine eye offend thee, pluck it out: it is better for thee to enter into the kingdom of God with one eye, than having two eyes to be cast into hell fire: [48] Where their worm dieth not, and the fire is not quenched.
See All..., Philippians 1:21-23
See All..., Revelations 20:4
See All..., Hebrews 12:1Wherefore seeing we also are compassed about with so great a cloud of witnesses, let us lay aside every weight, and the sin which doth so easily beset us, and let us run with patience the race that is set before us,
See All...).

Another reason that we know these Old Testament saints are still alive is the fact of the resurrection. These men and women also had the same hope as believers do now of a bodily resurrection.

David definitely had this hope.

Psalms 17:15As for me, I will behold thy face in righteousness: I shall be satisfied, when I awake, with thy likeness.
See All... – "As for me, I will behold thy face in righteousness: I shall be satisfied, when I awake, with thy likeness."

Again David speaking,

Psalms 71:20Thou, which hast shewed me great and sore troubles, shalt quicken me again, and shalt bring me up again from the depths of the earth.
See All... - "Thou, which hast shewed me great and sore troubles, shalt quicken me again, and shalt bring me up again from the depths of the earth."

Also see the hope he had of seeing his dead son again in 2 Samuel 12:23But now he is dead, wherefore should I fast? can I bring him back again? I shall go to him, but he shall not return to me.
See All.... Job also had this assurance that the grave was not his final resting place when he hinted at what his glorious future would hold.

Job 14:14If a man die, shall he live again? all the days of my appointed time will I wait, till my change come.
See All... – "If a man die, shall he live again? all the days of my appointed time will I wait, till my change come."

Again he reiterated his belief of a resurrected bodily existence in -

Job 19:26-27 [26] And though after my skin worms destroy this body, yet in my flesh shall I see God: [27] Whom I shall see for myself, and mine eyes shall behold, and not another; though my reins be consumed within me.
See All... – "And though after my skin worms destroy this body, yet in my flesh shall I see God: Whom I shall see for myself, and mine eyes shall behold, and not another; though my reins be consumed within me."

Isaiah also believed in the resurrection when he prophesied of this future event for the nation of Israel.

Isaiah 26:19Thy dead men shall live, together with my dead body shall they arise. Awake and sing, ye that dwell in dust: for thy dew is as the dew of herbs, and the earth shall cast out the dead.
See All... - “Thy dead men shall live, together with my dead body shall they arise. Awake and sing, ye that dwell in dust: for thy dew is as the dew of herbs, and the earth shall cast out the dead."

Daniel 12:2And many of them that sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt.
See All... - “And many of them that sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt.”

In fact, a hope in the bodily resurrection was not limited only to the believers after the Cross but is found through out the Bible in every dispensation and is one of the great promises of comfort and hope for believers of any age (1 Thessalonians 4:18Wherefore comfort one another with these words.
See All...).

We can also add the testimony of Jesus Christ Himself to the evidence of these saints existence when He referred to the patriarchs being present during his future Millennial Reign on the Earth.

Matthew 8:11And I say unto you, That many shall come from the east and west, and shall sit down with Abraham, and Isaac, and Jacob, in the kingdom of heaven.
See All... – "And I say unto you, That many shall come from the east and west, and shall sit down with Abraham, and Isaac, and Jacob, in the kingdom of heaven."

At the Mount of Transfiguration we also see Moses and Elijah appear and visit with the Lord.

Matthew 17:1-3 [1] And after six days Jesus taketh Peter, James, and John his brother, and bringeth them up into an high mountain apart, [2] And was transfigured before them: and his face did shine as the sun, and his raiment was white as the light. [3] And, behold, there appeared unto them Moses and Elias talking with him.
See All... – "And after six days Jesus taketh Peter, James, and John his brother, and bringeth them up into an high mountain apart, And was transfigured before them: and his face did shine as the sun, and his raiment was white as the light. And, behold, there appeared unto them Moses and Elias talking with him."

In fact, when Jesus was faced with the religious leaders (Sadducees) of his day who didn’t believe in the resurrection or an afterlife he refuted them for their false doctrine by explicitly referencing Old Testament saints.

Matthew 22:31-32 [31] But as touching the resurrection of the dead, have ye not read that which was spoken unto you by God, saying, [32] I am the God of Abraham, and the God of Isaac, and the God of Jacob? God is not the God of the dead, but of the living.
See All... “But as touching the resurrection of the dead, have ye not read that which was spoken unto you by God, saying, I am the God of Abraham, and the God of Isaac, and the God of Jacob? God is not the God of the dead, but of the living.”

Paul also testifies that this hope was one of the sustaining strengths that helped these saints persevere and faithfully endure the great trials of persecution that they suffered:

Hebrews 11:13-15 [13] These all died in faith, not having received the promises, but having seen them afar off, and were persuaded of them, and embraced them, and confessed that they were strangers and pilgrims on the earth. [14] For they that say such things declare plainly that they seek a country. [15] And truly, if they had been mindful of that country from whence they came out, they might have had opportunity to have returned.
See All... “These all died in faith, not having received the promises, but having seen them afar off, and were persuaded of them, and embraced them, and confessed that they were strangers and pilgrims on the earth.” For they that say such things declare plainly that they seek a country. And truly, if they had been mindful of that country from whence they came out, they might have had opportunity to have returned.” But now they desire a better country, that is, an heavenly: wherefore God is not ashamed to be called their God: for he hath prepared for them a city.”

In fact, these same Old Testament saints that Paul is referring to in chapter eleven are said to be the witnesses in

Hebrews 12:1Wherefore seeing we also are compassed about with so great a cloud of witnesses, let us lay aside every weight, and the sin which doth so easily beset us, and let us run with patience the race that is set before us,
See All... - “ Wherefore seeing we also are compassed about with so great a cloud of witnesses, let us lay aside every weight, and the sin which doth so easily beset us, and let us run with patience the race that is set before us.”

In light of this and other scriptural evidence, I believe that there can be no doubt as to the eternal existence of the Old Testament saints and that we will shall live and fellowship with them together throughout the ages to come.