Scott Williams and O'Driscoll came together in a huge clash back in 2014.
Ireland will open their Six Nations campaign in the Aviva Stadium against Wales on Sunday, and it's undoubtedly going to be a physical encounter.
One man who knows that better than many others is former Ireland captain Brian O'Driscoll, who remembers one particular occasion when there was a huge collision between himself and Wales' Scott Williams.
It being Throwback Thursday, O'Driscoll shared an image of the aftermath of that hit back in 2014, which left the Ireland legend with quite a bruise all across his "hoop", as he puts it himself.
A photo posted by Brian O'Driscoll (@13rianodriscoll) on
For his part, Williams didn't come off all that much better, and he responded on Twitter with a picture of his x-ray, as he also had to come off the field with a shoulder injury and required surgery afterwards.
Never been in a car crash but that can't have been far off one!! Came off second best though...... https://t.co/TULQp0TPQn— Scott Williams (@ScottWilliams_1) February 4, 2016