"God, after He spoke long ago to the fathers in the prophets in many portions and in many ways, in these last days has spoken to us in His Son" (Heb. 1:1-2).
The Old Testament is but a sample of what is revealed in the New Testament.
When Jesus said, "Do not think that I came to abolish the Law or the Prophets [the Old Testament]; I did not come to abolish, but to fulfill" (Matt. 5:17), He was affirming that Scripture progressed from promise to fulfillment, from partial to complete. We call that progressive revelation.
For example, the Old Testament anticipated Christ's coming; the New Testament records His coming. The Old Testament writers didn't understand everything they wrote because it didn't always apply to their day. That's why Peter said, "As to this salvation, the prophets who prophesied of the grace that would come to you made careful search and inquiry, seeking to know what person or time the Spirit of Christ within them was indicating as He predicted the sufferings of Christ and the glories to follow. It was revealed to them that they were not serving themselves, but you, in these things which now have been announced to you through those who preached the gospel to you by the Holy Spirit" (1 Pet. 1:10-12).
Progressive revelation doesn't at all imply that the Old Testament is inaccurate. The distinction isn't in the rightness or wrongness of the revelation, but in its completeness. Just as a child progresses from letters to words to sentences, so God's revelation progressed from types, ceremonies, and prophecies to final completion in Jesus Christ and the New Testament.
Though incomplete by New Testament standards, the Old Testament is nonetheless fully inspired by God. That's affirmed often in the New Testament. Peter tells us that no human writer of the Old Testament wrote of his own will, but only as he was directed by the Holy Spirit (2 Pet. 1:21). Paul added that "all Scripture is inspired by God and profitable for teaching, for reproof, for correction, [and] for training in righteousness" (2 Tim. 3:16, emphasis added).
The Old Testament isn't all of God's truth, but all of it is true. And as you progress from the Old to the New, you see God's character and redemptive plan unfolding in greater detail.
Suggestion for Prayer
Praise God for the fullness of revelation you enjoy in Scripture.
For Further Study
Memorize 2 Timothy 3:16-17.