I honestly can't remember what I used to login to Ubuntu Forums, but I am quite sure it was OpenID. Should I be concerned?
Since OpenID login is always processed on the issuer side (for example, if you use OpenID you got from Launchpad, then Forums will contact Launchpad and it is Launchpad who processes your authentication), the Forums do not keep your OpenID password (they don't need it at all).
Therefore, all the attackers can get is your OpenID login, but not the password, since it's not actually there.
Also, just for completeness, according to this official announcement, only the Forums are affected, no other services are.
With OpenID, your password is only given to your identity provider, and that provider then confirms your identity to the websites you visit. Other than your provider, no website ever sees your password, so you don’t need to worry about an unscrupulous or insecure website compromising your identity.
The identity provider is for instance Google and the website you visit is UF.
So yes you should be safe.
Generally speaking (to the best of my knowledge, at least) when using an Open ID account to log in, the authentication is handled purely by the external website (so for example, if I were to use Facebook to log in here, the authentication would be handled purely by Facebook and not by Ask Ubuntu). The other site only hands over a unique identifier that tells the Ubuntu Forum / Ask Ubuntu / whatever that you are you, and doesn't pass on any of your login details.
So the passwords stored (+ hashed and salted) by the Ubuntu forum would be for local accounts only (as mentioned in the "What we know" section of the current maintenance page)
Of course if you're still concerned about it, it wouldn't hurt to change your passwords - and for peace of mind it would probably be a good idea to do so anyway - but as far as I am aware unless the service you used to authenticate your Open ID was breached (Google, Facebook, etc) there shouldn't be too much cause to worry.
See this page on the OpenID website for further details.
If you really used OpenID: No.
The login process is executed between your OpenID provider and you. The services that allows you to use OpenID doesn't even see your password! There is no possibility how ubuntuforums could have stored your password.
The following resources might help you to understand how OpenID works.