Can I talk about articles I'm refereeing?
There are different confidentiality requirements involved, some to protect the authors, some to protect the referees.

A) As a referee, you absolutely must not reveal the content of an article you are refereeing to others. In many cases, the paper will be available on the arXiv anyway, and you can speak about the contents of the arXiv preprint, even if you are refereeing the paper, but not about anything you couldn't have learned outside of the refereeing process.

R) It is not customary to reveal oneself as a referee of a specific paper, but there is no strong consensus on how strict this should be handled. It is quite certainly ok to mention that one is refereeing for a specific journal (many people list the journals they have refereed for in their CV).

To your detailed questions:


When discussing the article, do not mention that you are refereeing it. Be careful not to mention details exclusive to the submitted version and absent from the preprint.
When ask by your office mate, it is fine to say that you are refereeing. They should not pry for details. If they know which article you are refereeing, do not mention the journal. If not, you can mention the journal. Better not name the editor.
As mentioned in the comments, it is typically better if referee reports are independent. Do not discuss an ongoing referee report with your co-referees.
Some people do reveal themselves as referees to authors. In general, I would advise against it though.