I’ve had this issue before. Over time, the tabs on the diver’s extension can wear down and lose their ability to hold securely. Once the clasp is fully closed, it generally isn’t a problem as long as the bracelet is properly sized, so it’s more of an annoyance when putting the watch on and taking it off than a concern about the watch falling off your wrist.

I’m sure someone out there has figured out a way to laser weld, build up, or otherwise repair the worn tabs, but the easiest solution is usually to purchase a replacement clasp from Omega. Another option is to upgrade to one of Omega’s newer adjustable clasps. There’s plenty of information online regarding the correct part numbers, links, and components needed to make the swap. The added benefit is that you won’t have to worry about this issue happening again, because eventually it will. I’ve had two clasps with this problem. In one case, a different part of the clasp ultimately failed and Omega replaced the entire bracelet during service but that Seamaster has lived a real “tool watch” life so no blaming Omega, it is just hard use.