Need a service on my 145.012

They will return the original parts to you, but there is no guarantee that they will follow your instructions. They will determine what they think is appropriate to properly service the watch. If you want to be sure to preserve the vintage character of the watch, you are better off seeking out and using one of the many independent watchmakers recommended here on OF.

321's are "Bienne only", so there's no doubt it would go to Switzerland if whatever service center you take it to is properly following procedures.

As for asking Omega, I suspect the only answer you will get is "send it in" so they aren't going to tell you much without seeing the watch.

As for Rolex following instructions...well let's just say that like Omega, they have their policies. If you send a watch to either brand asking "don't replace the hands" and they didn't replace them, that means there's nothing wrong with the hands, and they wouldn't have replaced them anyway. If they feel that the hands may cause a problem, you won't have a choice, as they will replace them. Either that or if you are lucky, they will send it back without service...but even that is a risk...all it takes it for someone not to get the message and the value of your watch is greatly reduced. At the very least, Omega will return your parts, but Rolex throws all the tritium dials and hands they remove in a big drum, and it's sent for disposal, so you don't get anything back.

If you value originality, for mid-tier brands like Omega or Rolex, the last place you want to send it is the brand service center.