The initial optimism over O'Brien's shoulder injury was dampened when he suffered an infection following surgery.
That set his return back by two weeks and forced him to forget any chance of taking to the field before the end of the season.
And that means the Irishman will now turn his attention to next season and is aiming to get back to full fitness before the campaign.
"I won't play till next season. There's no point in trying to rush back now before the end of the year or the summer tour," said O'Brien.
"It's probably a thing of starting a pre-season in a week or two, getting in good condition and getting ready for next year.
"It's looking good and I'm probably ahead of schedule considering everything that happened. I got a little infection in it a week after the op and had to have it cleaned out again.
"That set us back a week or two probably rehab-wise."
The injury was sustained when the British & Irish Lion was playing in his side's 19-6 RaboDirect PRO12 win over Ulster.
It was only O'Brien's second league appearance of the season after a series of injury problems that have hampered consistency.
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