The network declined comment on his condition or whether he was hospitalized.
But a person close to the production said O'Brien was being examined at a hospital. The person, who lacked authorization to discuss details of the matter, spoke on condition of anonymity.
O'Brien issued a statement through NBC that indicated he was treating the incident lightly.
"Last thing I remember I was enjoying the play with Mrs. Lincoln, and the next thing I knew I was in bed being served cookies and juice" O'Brien said in the statement.
The 46-year-old comedian took over the "Tonight" host's job from Jay Leno in June.
O'Brien's accident recalled a rare absence by Leno that came toward the end of his 17 years with the show, when a high fever and unspecified illness caused him to check himself into a hospital for a day.
NBC canceled two episodes of "Tonight" when that occured in May. Leno's new daily prime-time comedy show debuted this month.