The large one i put on the sump plug, but it was the wrong size and caused the plug to leak. The plug does have a seal in the head of the bolt, which is not possibly to replace. The only way i could stop the plug from leaking again was to renew it! It only cost a few quid from ford.
The mondeo does use 2 different sizes of sump plug (unsure why), so they give u an o ring for either size. Just bin them, refit the plug with the same seal, and if it leaks then get a new plug from ford :thumbup:
if its a motocraft/ford oil filter then the 2 o-rings are for the oil filter base, big one for the main housing itself like you say & if you look in the middle of the filter base from the underside/outside there is a small metal bit, this unscrews & the small o-ring fits on there :L
its the drain plug o-ring and its fiddly to change with the metal being soft & oily so it may not be worth the effort unless it starts weeping oil from that plug, last time i actually usd that plug to drain the oil from the filter it did not drain much & i still got covered when i removed the rest of the filter :}
i never got any sump plug rings with the oil filter i bought, just the 2 oil filter related ones but the info on sump plugs is bang on as the computer insisted my mota had one type of sump plug and the parts bloke got a little arsy about it when i told him thats nothing like what i removed from my mota ( it was long & thin that he handed me & the one on my mota was shorter & chunky ) took great pleasure in prooving him & his computer wrong when i went back with my old sump plug after doing the oil change with the sump plug i picked from the range of sump plugs he brought out in the end