I'm not listed on the note of my ex-husband's mortgage but still on deed. Is it OK to buy a new house?

Remarriage isn't a problem.  You're legally single.

The fact that the mortgage isn't on your credit report might only mean that your ex-husband is current with the payments, which is a good thing.  You should remember, though, whether or not you signed the mortgage and the note, and I suspect that you did, unless he put you on the deed after purchasing the home on his own.  If he starts missing payments, you might very well see this on your credit report, and it's an all-too-common problem these days.

If the divorce decree requires him to refinance, you might want to press that issue, just to prevent future problems, unless you're that confident of your ex's financial stability.  In the meantime, you've go an accurate credit report, it seems, and there's no reason you shouldn't take advantage of it.  If the ex's mortgage creates a problem later, and you're accused of not having disclosed it, the decree will solve the non-disclosure issue.