For an Optima you want a 34R IIRC. That is a red top with reversed terminals. Unfortunately most of the local suppliers in my area do not carry the reversed terminal version, so you have to buy online.
Another choice is a Group 48 battery from just about anywhere. It is shorter, but there is a hole properly located for the battery hold down with this battery. The Group 48 that I got came with a vent kit to vent outside the car. MUCH cheaper.
A forum community dedicated to BMW owners and enthusiasts. Come join the discussion about Bimmerfest events, production numbers, programming, performance, modifications, classifieds, troubleshooting, maintenance, and more! Bringing the BMW community together.