The iPhone 7 is a better phone specs wise, the processor is faster - although most flagships are fast enough anyway, the camera will probably be far better, the battery life will most likely be better, iOS is far better optimized for the hardware it runs on, apps generally run better on iOS, and if you don't mind the fact that iOS is closed and not as customize-able, then it would probably be better for you. Not to say that the OP3 is slow, because it isn't, it is one of the fastest android phones. Also I have had no problems with my OP3.
The iPhone is quite an expensive phone - possibly even overpriced, especially compared to the OP3, so it really comes down to whether you want to spend the extra money on it.