Yes, DP4 for OSX has mixing capabilities. DP4 is a full-featured DAW that has pretty much any audio processing things you would want. EXCEPT...it can't handle VST plug-ins without a wrapper. If you don't know what that means, then you can safely ignore it.
DP4 is not the cheapest way to get some audio mixing. I mentioned it because DP4 and the 828 firewire interface are both made by the same company, MOTU. MOTU's gear works really well together with other MOTU gear. I use DP3 with an 828 and it's flawless. However, if you're willing to take a bit more risk I am sure there are plenty of other programs that'll work well with the 828.
Hope that helps.