First off, once that oscar reaches full size, and along with the plecos, then you will have a fair bio load going on in that 125. So, if you want to add more fish (and I agree, that tank is crying out for more fish/decor) then i think you need to stay away from fish that are going to get large, your bio load will eventually be crazy. So, smallish fish it is, but, will they end up as dinner for your oscar? Depends on the choice of fish.
I'm a great fan of filament barbs. Check out pictures of them on line, they are stunning, especially when they develop their "filaments" on their dorsel fin and the males start showing off. They will certainly give you more movement in that tank, they are constantly on the go. As a result of their movement and speed i'd be surprised if your oscar could catch them! They grow to about 6-7" and you would need to get six minimum, they look far better in multiples. They are plant destroyers though being barbs, but you don't have any!
Admittedly i'm a little biased because I have them, but at least give them a thought, look them up, I think they'd give you the movement, a bit of colour, and a bit of a wow factor too in that currently very drab looking tank.
As they grow though you are going to have to be on your game regarding water changes due to total bio load.