If a student is good enough to be admitted RD, where the odds are much steeper, and has committed to attend (and is therefore “serious”) by applying ED, why would Oberlin deny them rather than just defer them?
Maybe it’s just an acknowledgement of how random and labor-intensive the process is. Oberlin only wants to carry so many applications forward to the RD round, so they deny a bunch of ED applicants who are acceptable but interchangeable with some of the kids they anticipate they’ll see in the RD pool and admit then. The benefit to Oberlin of clearing their desk in this way outweighs the modest yield risk they take by knowingly removing from consideration a number of kids who were committed enough to Oberlin at one point that they applied ED there. It sure sounds odd, though.
I can understand it better at ND, where there’s a lopsided applicant pool, plus EA is non-binding and therefore there’s no commitment by the applicant (unless they’re so good that it’s clear they decided to forgo an SCEA app at a tippy-top in order to apply to ND - in which case ND might choose to admit them EA). One can see why ND might deny a bunch of very-good-but-not-clearly-tops Midwestern Catholics early just because they know there are plenty more who’ll take an ND offer later. Also, I’m guessing that because of its niche status, ND cares less about yield than Oberlin.