O'Connor will join Connacht on a two-year contract at the end of the current season. The 6ft, 90kg out-half, who attended Limavady Grammar School, entered the Ulster Academy in 2006 and signed a professional contract with them in 2007.
O'Connor, who is 23, has 51 Ulster caps to his name and has scored 212 points for his native province.
He has represented Ireland at Under-19, Under-20 and 'A' level, and was also a member of the Ireland Under-20 squad, coached by Connacht's Eric Elwood and Dan McFarland, that captured the Grand Slam in 2007.
O'Connor's Ulster colleague McCrea, who plays on the wing, joined the Ulster Academy in 2006 and turned professional in 2008.
He has won 25 caps for Ulster and has represented Ireland at Schools and 'A' level. The 23-year-old Belfast High School graduate was a member of Ireland's 2008 Churchill Cup squad, scoring a try against the USA, and has three Ireland 'A' caps.
Meanwhile, the highly-regarded Naoupu will return to Connacht for the 2011/12 season. The 6ft 5in, 115kg back rower joined Connacht in 2009 before being lured away by the Kobe Steelers in Japan last season.
The 27-year-old made 14 appearances during his six-month spell at Connacht, and scored a try in the westerners' famous victory over Bourgoin in last season's Amlin Challenge Cup quarter-final.
Naoupu has represented the New Zealand Schools, Under-19, Colts and Sevens sides and will rejoin Connacht on a two-year contract.
CEO Gerry Kelly said: "We are delighted with the news that these players are joining Connacht. Mark McCrea and Niall O'Connor are both outstanding players and they will no doubt add an extra dimension to our back-line.
"Recalling George Naoupu from Japan is a big move for us. He was exceptional for Connacht in 2009 and a big loss to the squad when he moved last year.
"George is a dynamic ball-carrier and an inspirational player to those around him, so we are looking forward to his return."