Shoot me for not knowing.
Other People’s Property.
Property was always left as a little ambiguous in the song though…I think we were supposed to presume it meant “personal” property 
It also represents 2 other words. One rhymes with leanest, and the other rhymes with wussy.
In what context? I found:
Office of Polar Programs
Optional Payment Plan
Ontario Provincial Police
Office of Pesticide Programs
Other People’s Problems
Opposite
FCC Office of Plans & Policy
Object Push Profile (Bluetooth)
Obligation Phasing Plan
Octal Print Punch
Office of Patent Publication
Office of Physical Plant
Office of Policy and Planning
Office of Pollution Prevention
Official Party Poopers 
Off-load Preparation Party
Off-Peak Power
Opening Price Point
Operation Process Plan
Opposed/Opposite
Optical Path Protection
Ontario Provincial Police, of course.
I was always under the impression that it was “Other People’s Parts.”
I’m sure you’ve hit it right on the head,Ethilrist!
I’m down with Office of Polar Programs. Yeah, you know me.
Other People’s Pussy
Jesus…
O is for Other
P is for People
Scratch your temple
The other P? Well…that’s not that simple…
I always thought it was Other People’s Problems.
I thought it was Original Party People. Did I read that or imagine it?
In the context of the song by Naughty by Nature, it means either “other people’s pussy” or “other people’s penis,” depending on your sex and, I imagine, your orientation. The last words of each phrase aren’t mentioned in the lyrics, but clues are given which make them obviously those words.
On an MTV interview, the group that did the song said it was Other People’s Pussy, as in having sex with another man’s woman.
I’m down with OPP
Yeah, you know me…
Hee, still remember them playing it at junior high basketball games…the ‘adults’ sure didn’t know what it stood for.
Bingo you get the prize.
Orleans Parish Prison
I’m down with Jarmusch and Ice-T.
Being down with Naughty by Nature is like being hep to Lawrence Welk.
Let me once again mention www.rapdict.org, an invaluable source for hopeless crackers like me who fancy hip-hop.
It concurs with the above responses.
Is it just me, or does “penis” not acutally rhyme with “cleanest of leanest”. The “t” at the end of those words just makes it impossible for my brain the rhyme them. I suppose said quickly, and in a song, it could pass, but still…