To tell the sex of an Oscar? Why not just ask!!! :
"Not easily distinguishable. Sex can be determined by examination of the mature oscar's vent. Females have a more rounded 'appendage' then the males (of course this information is useless unless you have two fish of the opposite sex to compare). Females grow smaller than their male counterparts."
"Sex can also be told be taking a net and skimming the top of the water. The male oscar will turn sideways and hold its gound where the female will turn sideways and back off slowly. This usualy only works on 6+ inch oscars."
from www.fishprofiles.com
http://www.fishprofiles.com/profiles/cichlids/oscar.asp
Of course I will use the disclaimer that this is only used for the "purposes of research, criticism or review, as permitted under copyright legislation" and I mean really, Im not making any money off this anyways and it certainly does not benefit me in any way. You can get it free anyways off any website but why re-invent the wheel. This information is to be a help anyways.
Whew! Just had to vent for a bit!