I want to grab a new domain name for a site I look after but many of the best word combinations are taken.
Is there any legal issue with using the word official in a domain name?
When it comes to legal questions you should always ask a lawyer.
That said, I think you're fine to do what you describe as long as you do not use the name of any professional, professional association or regulatory board in the domain. Additionally, you should let visitors know that you are not endorsed by the same and refrain from using any of their copyrighted / trade marked logos or other media.
Its not just copyright or trademark violations that you need to worry about. Also consider a user that had a bad experience with a dentist that they found using your web site. You would be amazed how easy it is to find a lawyer to sue someone for just about anything.
We get back to talking to a lawyer, who could help you write a suitable disclaimer and make sure that you stay out of trouble.
The only problem I can see is that some people might think it is the "official" site (e.g the site run by the profession's governing body), so they might think that your site has more authority than it actually has. In the extreme case that could be construed as misleading your target audience into thinking the site was something it isn't.
I suppose it all depends on what the domain name actually is.
A similar issue is with calling yourself an "institute". I used to work for an "institute" - which was really just a company selling educational products that had a very clever MD who called the company an "institute" instead.
In Australia there's nothing illegal in calling yourself an "institute" (immoral, perhaps) but your milage may vary in your local country.