in a 125 g, you could keep 2 or 3 oscars, 3 is a big maybe though as they might fight or not
aquascaping wise, large rocks like slate, driftwood and sand work well will oscars. plants will be torn up and anything small will be moved around in time. oscars like to dig.
as far as other fish, plecos, clown loaches, severum, convict would work well with the oscar. basically something big enough to not be eaten by the oscar, and nothing overly agressive as oscars are mild tempered and not all outgoing agressive. oscars will eat whatever fits in there mouths (which is large for there size) and more! they're always hungry and eat as long as you'll drop food. they also beg for food when they see you.
in my 90 gallon, i have 1 oscar, 3 clown loaches and one common pleco
water should be soft and acidic, although oscars will do fine in pretty much any kind of water. i put peat in my filter to help get the soft acidic water.
you'll need massive filtration too. a big canister filter and a big power filter like an aquaclear 110 (You'll want the filters to turn over at least 1100 gallons an hour) (i have a filstar xp3 and aquaclear 110 on my 90 gallon and i'm thinking of setting up a second aquaclear) as oscars are extremely messy. they spit out half of what they eat. my 3 inch oscar is very messy and yet so small, i can't imagine when he'll be 12 inches!
all in all, he's my favorite fish!