One little word can mean so much. Yes, any man is going to want you to say “oh” a whole lot during the actual deed, if you utter it any time before or after, it typically doesn’t mean anything good. If the clothes have come off, it sounds like you’re not impressed with his body but you’re trying not to say anything outwardly mean. If you say it after sex, again, it comes off like you just had a terrible time but you’re pretending that you didn’t.
Two people may sexually connect and it happens to be a dud. That’s fine. If you’re planning to have regular sex with this man, it may be worthwhile to express your needs and desires to him and see if the sparks fly. However, if it was just a one-night thing, there’s no reason to give the guy a play-by-play of how he can do better. It’ll just deeply shake his confidence.