What is the purpose of the second, smaller O-ring provided in the filter kit? Is it for the check valve inside the filter housing or for the drain plug on bottom of the filter housing? Asking for a friend.

 

what year/engine?

The big o-ring is for the filter housing that fits into a slot and the small one is for the oil filter cap plug. You should also use a fresh crush washer (usually copper or aluminum) for the drain plug but typically those are sold separately

 

It is a 2018 XC 90 T6 R-Design

the filter comes with two O-rings pre installed. So this leads me to believe this small washer is for the silver drain plug on the bottom of the filter housing.

I noticed in another post that they said it is for the check valve inside the filter housing.