O'Toole, 24, agreed a three-year deal with the Pirates on Tuesday following a successful loan spell with the club last season.
The former Watford man will be reunited with former U's boss Ward, and his manager is backing him to become an instant hit.
"We are all delighted to welcome John-Joe to the club and see all our patience pay off," Ward told the club's official website.
"It has been no secret that we wanted to bring John-Joe to the club and we have worked very hard to get him here.
"His style of play gives confidence to the people around him. We are an extremely young squad, and although he is only 24 he has had experience of playing at a variety of different levels.
"He has a lot of charisma and his experience and enthusiasm will rub off on those around him."
- Sports & Recreation