OPP rank is actually short for 'opponent rank' or 'opponent ranking'. It's a variable statistic that almost every fantasy football website will use to help you determine who you should start each week, based on your matchup.
How it Works
Say you have Aaron Rodgers playing quarterback for your team. There's a couple of ways the OPP rank statistic may be used. Opponent rank could mean the rank of the defense that Aaron Rodgers will be going against this week, or it could also mean the rank of your fantasy football opponent's quarterback. If your opponent has Tom Brady, your opponent rank would be very high since Tom Brady is an elite quarterback.
If the website uses the other way of measuring opponent rank and the rank is very low, it would mean your player is playing against a weak team or player, or at least one that is worse than your team or player. Use this to your advantage!
That may have seemed like a lot of words at once, so let's break it down even further.
Opponent Rank Style One
A style in which 'opponent rank' is judging one of your players against a player on your opponent's roster. QB-QB, RB-RB, etc.
Opponent Rank Style Two
By the Numbers
Most websites will provide the OPP rank for you as a ranked number, like 1st or 17th. If Aaron Rodgers is on your roster and he will be going up against the Giants that week, and the OPP rank next to his name says 20th, that means that out of the 32 NFL teams, the Giants' defense is 20th best in the league at shutting down the quarterback.