Actually, the larger ones are for sealing the female ends (the ones that have the flange plate) to the IC tubes, not for making the seal where the tubes detach from the intake and IC. Those ends will unscrew from the IC tube and the O-ring goes there.
It's hard to see the O-ring in the pic, but there is a receiver groove in the donut shaped part. This seals the part to the end of the IC tube. There is also a bigger O-ring that seals
the inside nut on the top to the threaded end of the top outlet. That part is a left-hand thread. The pieces on the IC tubes are standard right-hand threads.