The second generation Oldsmobile Omega was sold from 1975 to 1979. It was based on the X Body platform by GM, and it was the top model of that platform. It was similar to the Nova, and they were both rear wheel drive cars. The Omega was sold as a sedan, coupe, and hatchback.
The final generation Omega was based on the Citation. The only engine options now were the Pontiac Iron Duke and the LE2 V6. These cars were sold from 1980 to 1984.
There was a first generation Omega that was only sold from 1973 to 1974. It was also based on the Nova, but the Nova was in the midst of changing platforms from a little pocket rocket muscle car sleeper to a more economical compact car.
You can still sometimes find an Omega on the used car market. The Citation-based models tend to wear down quickly, although I once owned a Citation that made it to six figures in mileage and ran like a charm every day I owned it, so it's really a game of chance I suppose. The earlier Omegas tend to be more desirable, especially if they have low mileage. They are not quite considered classics, but they were sort of made in that transitional period from muscle car to 80's econobox crap. The best place to buy a used Omega or parts for an Omega is on eBay.