Omega automatic, missing knob?

You likely also need a stem which the crown (knob) screws onto, And from the general appearance of the watch, other repairs. Your watchmaker can look after that for you. Or you could make a very expensive repair out of by trying to do it yourself.

As mentioned above you really need a professional watchmaker to open the case and get the serial and movement number, then people can give you a lot more help / you can put a post up with the actual watch reference. Someone may well have a pristine example and or reference material. You can also search it on the Omega website and even order and extract from their archives about the actual watch (not commercially sensible for this one but nice to have)
As suggested by @Widows Son share your approximate location and people may recommend local watch makers.
Good luck / it is a nice memento to keep in the family for generations.

Also you can edit your thread title to missing crown if you like now :0)

Just being nosey what was your fathers occupation - I would guess this watch is from the forties or early fifties ( The serial number will confirm that)