OK, This site indicates that there is some controversery about this in OK. See:
Common Law Marriage in Oklahoma
Apparently, the courts have upheld case law that basically states if it looks like a duck and quacks like a duck it's a duck. So a couple that says they are married and acts like they are married may be considered married. One of the requirements is that the:
The couple publicly considers themselves to be husband and wife.
I would say that filing a tax return as married would be a public statement that they consider themselves married.
Common law marriages are becoming very much the exception as states are closing those loopholes. So we may be forgiven about forgetting about that possibility.
Now the question becomes how does one end a common law marriage. For that I'm going to move this back to Family Law.
I found this on a site about Colorado, but it may apply to OK as well:
Myth 6: So what if I am considered common law married, I can simply “annul” the marriage, right?
Wrong. If you are held to be common law married, you will have to go through a full-blown dissolution of marriage proceeding.
So it would appear that if you declare yourself married under common-law, then you have to formalize the dissolution of that marriage.