You do know that Paul preached from the Old Testament and his doctrine was checked by the Bereans? and they validated his teaching against scripture:
Ac 17:11 These were more noble than those in Thessalonica, in that they received the word with all readiness of mind, and searched the scriptures daily, whether those things were so.
If you cannot find Paul's doctrine in the OT, then you don't know how to read the OT.
You do know that Paul was endorsed by Peter?
2Pe 3:16 As also in all [his] epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as [they do] also the other scriptures, unto their own destruction.
If you reject Paul, you reject Christ.