If you look at the strainer you'll see it has 4 small legs, these touch the bottom of the sump and keep it in place and also serve as the gap for the oil to flow through before entering the strainer, they are high enough so that any sludge at the bottom of the pan doesn't get sucked up into the strainer, until of course it gets deep enough for it to do that, in which case you've already got severe problems. So no need for an oring.