Omega probably has a specific part number for the seal that you need. If you don’t have access to Omega parts. The seal you refer to is likely made of red plastic. I gather you’ve had the case back off, and the red plastic seal stayed in the case. When that happens, most people try to put the case back, back on! If this happened with your watch, you have just demolished the gasket! The correct procedure is to remove the case back, if the seal remains in the case, it must be removed from the case, and positioned on the caseback in the same orientation. I know I’ll wear a brown helmet for suggesting you re-use the gasket. But what the hey! You are now in a position where you need to subscribe to the services of a watchmaker. Someone who has an assortment of these seals. Best of luck!