By all means, and we need to start at Hebrews 11:39,40: And these all [OT saints], having obtained a good report through faith, received not the promise: God having provided some better thing for us, that they without us should not be made perfect... 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... But ye are come unto mount Sion, and unto the city of the living God, the heavenly Jerusalem, and to an innumerable company of angels, To the general assembly and Church of the firstborn, which are written in heaven [NT saints], and to God the Judge of all, and to the spirits of just men made perfect [OT saints], And to Jesus the mediator of the new covenant, and to the blood of sprinkling, that speaketh better things than that of Abel. (Hebrews 11:39,40; 12:1,22-24).
So what do we learn from these verses?
1. God would perfect the OT saints at the same time as He did the NT saints.
2. The OT saints are called "so great a cloud of witnesses".
3. Witnesses by definition are alive, awake, alert, and observant.
4. These are all living saints in Heaven, though they have passed on from this earth.
5. Then we are shown all the actual occupants of the New Jerusalem (which is in Heaven). Thus "the heavenly Jerusalem".
6. The NT saints are called "the general assembly and Church of the firstborn, which are written in heaven" because their citizenship is in Heaven.
7. The OT saints are called "the spirits of just [or justified] men made perfect" because they were all perfected after Pentecost.
8. The very fact that all these saints are in the presence of God and Christ in Heaven means that God is not the God of the dead but of the living.