A graduate of the Phoenix Ulster Academy, O'Connor returns to his native province having enjoyed a one-year spell with Connacht.
The 24-year-old out-half established himself as a first choice player in Galway and produced a number of fine displays for the westerners throughout the season.
The highlight of his campaign was the 9-8 win over Harlequins, in which he kicked all of Connacht's points to help record a first ever Heineken Cup win for the province.
During his time with Ulster, O'Connor made 51 appearances and scored 212 points. He will compete for the starting out-half position with Ireland Under-20 captain Paddy Jackson and South African international Ruan Pienaar.
Hooker Herring will arrive in Belfast from Western Province where he was involved in their Vodacom Cup squad. The 22-year-old South African enjoyed a brief stint with London Irish before returning to his native South Africa.
The 16st 3lbs forward 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.
Commenting on the signings, 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."
O'Connor is delighted to be returning to Ulster and feels his game has been improved by the stint in Galway, admitting: "I got a fair bit of game-time down there and I'd like to thank Eric (Elwood) 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 province, I thought it was a fantastic opportunity as Ulster are one of the best sides in Europe.
"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."