But O'Brien successfully completed 71 minutes of Leinster's interprovincial derby win over Munster on Sunday and was named man of the match.
And with his bout of ill health seemingly behind him, O'Brien hopes he can play his part in helping third-placed Leinster continue to climb the GUINNESS PRO12 table.
"Thankfully, it's sorted," O'Brien told the Irish Independent. "I don't know if it was scary but it was very frustrating.
"Leinster have been very good with me and not rushing me back. It's felt grand and I felt very good out there.
"I wanted to play my part for the team and do my bit for the performance.
"There's been a lot of frustration over the past couple of weeks, it was more so about that than playing with any freedom."
And Leinster head coach Leo Cullen said the whole place had been given a lift by O'Brien's return.
"It's great," Cullen said.
"Even in the last few weeks Seanie's played a big role in terms of driving the group. We see him as a huge leader for the team and definitely we've missed his presence out on the field.
"It's his experience that he has as well, the presence he has and the respect he has amongst his peers, it's just fantastic to have him back."
To buy tickets for the GUINNESS PRO12 Grand Final click here. Also follow us on Facebook, join the conversation on Twitter, sign up to our YouTube channel for extensive match highlights and sign up for our newsletter for regular updates on the GUINNESS PRO12.
Guinness PRO12 Leinster News
63x46.png){kind=logo}