I still don't know what server is that ( #bitcoin-otc is a channel, not a server/network ) so i can't help that much, they could require you to signup at website before you can use undernet/whatever services (chanserv / nickserv).
.. a quick search gave me the answer : that's probably
http://bitcoin-otc.com/ irc servers you're trying to use (on the freenode irc network then) ?
partial quote from the "must-read for all users" guide :
Authentication requirement
To enter orders, and participate in the rating system, you must register and authenticate to the Gribble bot. You can register using a bitcoin address, a GPG key, or both.
Bitcoin address authentication documentation (this is probably easier unless you're already familiar with GPG)
GPG authentication documentation
Authentication is used to maintain your identity on the order book and rating system when conducting trade on the the #bitcoin-otc IRC channel (#bitcoin-otc via web).
It is also a good idea to register your nick with nickserv, and turn on nick protection (set enforce), so that others can't use your irc nick when you're not around.
Of course, you don't need to register if you want to trade without using the order and rating systems - just come into the channel and talk to people. You will find it harder to find willing counterparties for trade if you don't register, though.
Have you followed those instructions ?
(specifically "you must register and authenticate to the Gribble bot") ?
If you indeed followed those instructions without any error, i'd suggest you edit this thread topic to clearly indicate a bitcoin-otc registration problem, providing more informations on the steps you followed and where it started to stop working "as awaited", you'll probably get dozens of useful replies then.
