It wouldn't hurt to start here:

https://www.omegawatches.com/vintage-watches

But unless you want to pay top dollar for vintage watches, I'd start off with something from the Geneva line for the simple reason that they are far more likely to be what they claim to be. One problem you my bump into is that looking for firsts has been pretty thoroughly done and you may well end up paying a bit of a premium. I'd also be a little cautious about the early Omega 'bumper' automatics as parts can be a bit of a challenge. If I were starting collecting vintage Omega I'd go looking for one of their 'thirty series' hand winding movements (which includes the 26x and 28x series) as these are really nice movements with good spares availability and excellent wear characteristics. Omega pricing is all over the place though so I'd spend a bit of time getting familiar with what you like and exactly what it looks like.

Good luck.