O-rings are typically made of silicon, EPDM, Buna-N (nitrile), or neorene. Each one has different advantages in terms of chemical resistance, thermal extremes, and abrasion. For keyboards, none of these factors matter. The only thing that matters is the softness measured in durometer (30A, 40A, 50A, etc). Just get whatever is cheaper in the softness you want. EPDM is cheaper than silicone.
And for the record, it's EPDM, not EDMP. It stands for ethylene propylene diene monomer rubber.