We all have the same instructions: Mat 28:18 And Jesus came and spake unto them, saying, All power is given unto me in heaven and in earth. Mat 28:19 Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: Mat 28:20 Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. Amen.
Paul was specifically sent accrding to the leading of the Holy Spirit. He soemtimes made plans only to be overuled by God. His mission was to preach the gospel to the Jews first ( as they were the children of the promise) and then to the gentiles.
As far as the Old Testament he preached the law fulfilled but there are many tenants of the Old Testament still valid such as salvation by faith, deliverance by faith, walking by faith etc. One dose not nullify the other but rather fulfills the other.