I can see that there are pipes in the ends of the strap. I've not mounted one of these, but from what I see the pin should be put through the case first, then the strap is mounted to the pin. You would have to put the strap on one end of the pin and tap the pipe onto the pin, then do the same for the other - might be able to do both if you can stand the watch on edge and line everything up somehow.
Note the if the pin and pipe don't have a good solid press fit, you may have to replace the pins. Not one of Omega's greatest designs for holding a strap on to be honest...
Hope this helps.
Cheers, Al