The US has published four secret memos detailing legal justification for the Bush-era CIA interrogation programme.
The fourth document, dating from 1 August 2002, was written by OLC lawyer John Yoo and signed by his colleague Jay Bybee.
It contained legal authorisation for a list of specific harsh interrogation techniques, including pushing detainees against a wall, facial slaps, cramped confinement, stress positions and sleep deprivation.
The memo also authorises the use of "waterboarding", or simulated drowning, and the placing of a detainee into a confined space with an insect.
Announcing the publication of the memos, Mr Obama said: "I believe that exceptional circumstances surround these memos and require their release.
"Withholding these memos would only serve to deny facts that have been in the public domain for some time," he explained.
But he also gave an assurance that "those who carried out their duties relying in good faith upon legal advice from the Department of Justice... will not be subject to prosecution."