Assuming owning mean individual custody of valuable asset, does owning cryptocurrency mean owning the tokens/coins? Does it mean owning a private key and owning a record in the "public ledger" and owning the software code associated with the particular cryptocurrency?
Ownership is a difficult concept and, in some contexts, irrelevant.
The answer you get from a lawyer or tax-adviser may differ from the one you get from a software developer.
Knowledge of a number gives you the power to spend money denominated in Bitcoin. Anyone else who knows that number has the same power. The Bitcoin network doesn't care who knew it first. It doesn't adjudicate ownership between multiple people who know a number.
I'm not sure you can own a number. If I know 12 do I own the number 12? Does that mean other people can't legally use 12 without my permission?. If I know that 12 is the number that opens a combination lock to a safe in my office is that a different sort of knowledge that confers ownership of 12 in some context? Does it confer ownership of the contents of the safe? Or is the legal ownership of the contents of the safe determined completely independently of knowledge of the combination? These are legal and philosophical questions. I think it may be best to avoid worrying about ownership and concentrate on keeping private keys secure.