You can output 2.1 from the Oppo via 3 analog RCAs if you use the bass management in the Oppo. If your amp is stereo only, however, it can only accept 2.0 via analog, regardless of the source.
Yes, you will be able to output 2.0 PCM full resolution (up to 24/48 and maybe higher depending on the title and the player) over spdif and the analogue outputs of both the PS3 and Oppo player. The players will mix the 5.0 soundtrack down into 2.0 for output so all dialog and effects (other than LFE) will be included. The Oppo is said to have the best 2.0 mix down of all the Blu-Ray players out there.