In a well-maintained tank oscars can grow as fast as an inch a month, so it could be very large pretty quickly.
Aside from that some of my other favorite green colored plants are java fern "Philippine," crypt lutea and crypt parva, Hemianthus micranthemoides ("regular" baby tears), Hygrophila corymbosa Kompakt, Staurogyne repens, and downoi.
Much however will depend on your lighting, ferts, and carbon source/pressurized CO2
. There are quite a few beautiful green plants if you decide to go low tech.