A graduate of the Phoenix Ulster Academy, O'Connor returns to the club having enjoyed a one year spell with Connacht. The 24-year-old established himself as the first choice out-half in Galway and produced a number of fine displays throughout the season. The highlight of his campaign was the 9-8 win over Harlequins, in which he kicked al the points to help record a first ever Heineken Cup win for the Irish province.
During his time with Ulster, O'Connor made 51 appearances and scored 212 points. He will compete for the starting position with Ireland U20 captain Paddy Jackson and South Africa international Ruan Pienaar.
Irish-qualified hooker Rob Herring will arrive in Belfast from Western Province, where he was involved in their Vodacom Cup squad. The 22 year old enjoyed a brief stint with London Irish before returning to his native South Africa.
The 16st 3lbs hooker joined the London Irish Academy in 2009 and made his senior debut against Northampton in May 2010. He will provide competition for Rory Best, Nigel Brady and Niall Annett.
Ulster Director of Rugby, David Humphreys, said: "We are continuing to build our squad depth and the additions of Niall and Rob will further enhance our ability to compete over the course of a gruelling season.
"Niall went away and gained a year's experience playing with Connacht and he has now come back to challenge Paddy and Ruan for the number 10 jersey. Rob is a typical South African forward who is very physical and is an accurate set piece operator. He is Irish-qualified and is a very ambitious individual who will thrive in our training environment," added Humphreys.
O'Connor feels his game has been improved by the stint in Galway. "I got a fair bit of game time down there and I'd like to thank Eric and his staff for that. I gained more experience and feel I have improved as a player over the past season. But when the possibility arose to move back up to my home club, I thought it was a fantastic opportunity as Ulster are one of the best sides in Europe," he said.
"It was great watching my mates do so well in the Heineken Cup last season. I'm looking forward to getting back to training next week to see them all again. I'm not thinking too far ahead but I'll get pre-season under my belt and then hopefully I'll get a run out in the friendlies and I'll take it from there."
Ulster have been drawn in Pool 4 of next season's Heineken Cup alongside Northampton Saints, Glasgow Warriors and Castres Olympique.