Galatians
Ephesians
Philippians
Colossians
Romans
Timothy
Titus
Thessalonians
On and on ... all Gentile communities. Paul quotes from the OT when writing to them, he quotes OFTEN. His epistle to the Romans discusses MOSTLY the law ...
Not sure what you're reading that youre getting this ...
The bigger question is where you are getting that the Old Testament had any relevance with anyone other than Jews. To the non-Jewish communities the Old Testament was as foreign as Chinese arithmetic. Recall that St. Paul often had to rebuke Judaizers for attempting to adhere to the Old Law.
In evangelizing the Gentiles St. Paul relied on three proofs for the deity of Jesus. The first that Jesus Christ alone among the pantheon of pagan gods was predicted, and relied upon the Old Testament's 450+ predictions. The weight of those predictions surpassed any argument of coincidence. The second was that He worked miracles, the foremost among them was the resurrection. The last was that nothing in the Gospel conflicts with human reason.
In this respect, the Old Testament was presented, but not so as to first make Jews of the Gentiles.