Nathaniel O'Brien (born March 3, 1983) is a Canadian competition swimmer who specialized in backstroke and butterfly events.

[1] He is a two-time national championship titleholder, and also, a member of the Pacific Coast Swimming Club in Victoria, British Columbia, under head coach and former Olympic swimmer Ron Jacks.

[2] O'Brien is the son of former swimmer Lisa Geary, who competed for the host nation Canada, and placed tenth in the women's 800-metre freestyle at the 1976 Summer Olympics in Montreal.

[4][5] O'Brien advanced to the semifinals from the morning's preliminary heats, with a qualified entry time of 2:00.49; however, he fell short in his bid for the final, as he emulated his mother's luck in tenth place, finishing the first semi-final at 2:00.13.

[6] In the men's 200-metre butterfly, O'Brien won the second heat by less than sixteen hundredths of a second (0.84) ahead of South Korea's Jeong Doo-Hee, with a time of 2:00.12.