Do Christians have to observe the Old Testament law and keep it?

No.

Matthew 5:17-18 is used by some people to prove that Jesus Christ did not break the Old Testament law, but
came to fulfill it.

However, that same passage shows that the Old Testament law is no longer applicable because Jesus Christ already fulfilled it.

He completed it.

Hence, we don’t have to keep the law.

Galatians 3:10 mentions that if we are under the law, we are under the curse.

Verse 13 also mentions that: if you are saved by faith, then you are not under the law.

Galatians 3:12 says, the law is not of faith and the Bible says, in verses 23 through 25, that the law was only at
the beginning to introduce faith.

But now that we are under faith, we are no longer under that schoolmaster of the law.