O-ring is just a shape. The actual product varies greatly in hardness, size and durability.
hardness: 100A or even 70A o-rings (on keycaps) don't really dampen anything at all; the impact is about as hard as without them. OTOH, 30-40A o-rings may feel quite mushy.
size: A thin o-ring in an SA keycap won't do anything at all, while a thick one on a Cherry keycap may even prevent the switch from actuating.
Cherry MX Silent switches, on the other hand, work with _any_ keycaps, hardly affect travel at all and come in a well-defined dampener hardness. Besides, they also dampen the upstroke a bit, which o-rings on keycaps obviously can't.