Use 'possessor' to indicate that someone legally owns an item or property. The possessor of the document was called to testify.
Pro Tip 2/3
Temporal Aspect
'Possessor' refers to having something at the present time and doesn't imply permanent ownership. He is the current possessor of the championship title.
Pro Tip 3/3
Transferable Possession
'Possessor' is suitable when discussing objects that can be given or transferred to others. As the possessor of the rare coin, she could sell it.