"Own" is more of aimplies legal termownership. If you "own" something, then it is your property. You can possess something without owning it by stealing it or borrowing it.
"Possess" is a very formal word. You would normally say "have" instead. "Possession" is much more common, and it's easier to see a difference between the "possession" and "ownership".
"Possession is nine-tenths of the law", but it's not legal ownership.