O'Connell, who has 101 Ireland caps, will join after the World Cup in September and October.
Despite worries about a troublesome shoulder, he passed a medical.
O'Connell's arrival will offset the loss of World Cup-winning locks Bakkies Botha and Ali Williams, who both retired at the end of the just-completed season.
South African-born Matt Stevens, 32, joins from the Sharks on a two-year deal after a 46-cap international career for England.
The trio join an impressive recruiting drive that has already seen the likes of All Black great Ma'a Nonu and American wrecking ball Samu Manoa agree to move to the reigning triple European champions.
Queensland Reds No10 Cooper has reportedly signed with Toulon, although since that announcement by the club in April, there has been speculation he might backflip.
There have also been reports that his Reds teammate James O'Connor is set to return to Toulon next season but the Australian Rugby Union only last week reiterated he has another year to run on his contract with the Queensland Rugby Union.
- AAP