The crown and pusher seals are checked and in this case, all are fine.  If they are not, the pusher seals can be changed to return water resistance  in most cases but, crown seal deterioration requires a new crown.

UK watchmaker showing service of Omega Speedmaster - case pusher seals check

The case and bracelet are cleaned and the movement re-fitted to the case.

UK watchmaker showing service of Omega Speedmaster - movement re-cased

Then the dust cover.

UK watchmaker showing service of Omega Speedmaster - dust cover fitted

A new case back seal is fitted and the case back tightened down.  With the bracelet re-fitted, the watch is finished and now goes into testing.

UK watchmaker showing service of Omega Speedmaster - finished watch