No .com domain exists using the standard single Latin letter o.
There has been talk and conjecture, and Verisign recently floated a strategy to offer o.com for auction to benefit non-profit causes.
Some companies already are trying to do business in that space, hoping a trademark on the still non-existent address might offer some priority. The firm Overstock.com, Inc. seems to have USA protection for various retail goods as well as vehicles, etc. -- claiming protection in many class codes!
Intellectual property & domain lawyer John Berryhill wrote about this process ten years ago (link) - calling this trademark registration process, when the domain is unavailable, as "phony" and deserving of contempt. Firms must be cautious of infringement.
