Saying the Father is the Only God is not the same as saying He is the Only True God. "Only True God" implies the highest authority above all other gods, not that no other gods exist. Second-century Rabbinical Judaism (Pharisees post-Temple) taught that the Father is the only God, while Jesus taught that the Father is the Only True God, which is a significant difference. The Psalms describe the Father as the king above all gods, a concept that post-Temple rabbis would consider polytheistic. Jesus referred to the Father as the "Only True God" because, in the end, the Father will be the only one to whom even the Son is subject, and everything will be subject to the Son. Therefore, the Father will always be God, all in all. This does not deny the existence of other gods but means that all other gods will ultimately be forever subject to the One True God, the Father.
