I'm guessing the problem is not the "bottoming out" sound, as you have o-rings installed. I think the noise which remains is the "topping out" sounds when the keys return to the unactuated position after release. There's not a whole lot that can be done about that, unfortunately. You can try things like:
- PCB mounted (non-plate-mounted) keyboard, such as a Cherry G80 series.
- Shelf liner or other dampening material under the PCB in the case to reduce keystroke volume
- Thicker keycaps, to further reduce vibrations which cause noise
Jdcarpe is absolutely right.
You guys after all this time still do not get that the problem with silencing Cherry MX is not the bottoming out noise.
O-rings, or even better, landing pads, take care of the bottoming out (as long as you are not hammering your keyboard).
The problem is that there is almost nothing we can do to silence the upstroke. Once you have silenced the bottoming out, the upstroke is louder than the downstroke. And it's a very annoying high-pitched sound (plastic against plastic).
The best method I know to silence the upstroke is to put non-conductive grease inside the switch. Not oil. The grease must have lower viscosity than oil. This limits the speed of the plunger on the upstroke and dampens the sounds rather well, but it also changes the feel of the switch. I find it acceptable to my taste, but I'm not sure you would like it.
Silencing the upstroke is possible on Topre switches, and the result is very good. Silenced Topre switches (on a Realforce) are so quiet that I can use it in a room where two people are sleeping (yes, I really do that). It works only with plate-mounted Topres (it's not as effective on the HHKB for example).