There are two types of 12 bolt wheels. The heavy wheels have a 13th hole for aligning the halves and the light wheels do not. You can not mix and match wheel halves, but you can mix wheels on a vehicle.
The o-rings are different between the two as well. For the light wheels they are readily available and cheap (I bought 10 for $10 on ebay). For the heavy wheels I paid over $12 a piece. It is possible to get them cheaper by buying o-ring stock and gluing it together, but I didn't want to take that chance.
You are correct that the o-ring is deformed. It starts out round and eventually takes a set. After a while they retain the triangular shape and can not be reused. You are supposed to replace them every time you break down the wheel, but if they are relatively new and still pretty round you can probably get away with reusing them.
Augi
