Not my area of expertise, but I've dug into this before...
If you're looking for an OEM option, your best bet might be to go to the Omega dealer/boutique. If you have the model number for the watch, someone here might be able to tell you if Omega still stocks official straps for this model.
Whether or not Omega still offer the official strap, you should be able to get an OEM deployant, if that's what you're looking for. Since most straps have some kind of taper, you're probably looking at an 18mm deployant for your 21mm strap.
If you're considering an aftermarket strap, you might look at Hirsch; their straps that pair with their own deployants. e.g.
https://www.hirschstraps.com/collec...h-speed-calfskin-deployment-watch-strap-black
If you're interested in going custom, I've read great things about https://dangerous9straps.com/ and https://www.aaronbespoke.com/
I hope this helps!
Best wishes
