A multi-channel amp is all that is required as long as you don't want any auto-EQ circuitry. The Oppos are exceptionally good units and running them direct into an amp sounds very, very good...
"The BDP-103 uses the same Cirrus Logic CS4382A (8-ch, 24-bit, 192khz) DAC that was used in the BDP-83 and the BDP-93 players, while the BDP-105 uses the ESS Sabre32 ES9018 (8-ch, 32-bit. 192KHz) DAC solution.
The BDP-103 will sound a little more open and warm, while the BDP-105 will be more accurate and detailed".
That's directly from Oppo to me. As far as processor goes, anything from 2 channel to multi-channel will work. Look at the specs on Oppo's site. Thing to consider is whether you need the 105 or will the 103 suffice. Is your gear high end, then go for the 105, if not the 103 will suffice and save you quite a few $$$.
I don't know what your budget is, but if your budget permits you may want to investigate the Parasound Halo A31 (retail $3000). The A31 is a rarity in that it's designed specifically to be a three channel amp as opposed to multi-channel amps that are also concerned with surround and/or back channels.