"The first week, we definitely over-trained on the Thursday and maybe the coaches were panicking a little bit about getting the information into us. On the first week [of the first Test], we had a triple [session] day, [the] lads' legs were heavy on the Thursday and we were playing the All Blacks on Saturday.
The Lions lost the opening test 30-15 at Eden Park before bouncing back to take the second test 24-21, handing the All Blacks their first home loss since 2009.
A victory in the third and final test would have secured the Lions their first series win in New Zealand since 1971. O'Brien says the squad were again overtrained on the eve of the final game.
"We did nearly a similar thing in the last week. So maybe it's more [from] a coaching point of view, in terms of taking lessons. Less is more sometimes on a tour like that, rather than trying to pick things up at the end of the week.
"There was probably no need for that but it's just the way it was managed. We had said it, at the time, and they pulled back a bit. But it's just about getting that fine balance between players and coaches and making sure the group is ready to rock."
O'Brien was forced from the field at halftime of the third test draw after coming off second best from a Jerome Kaino clear-out. O'Brien revealed to Irish site Sports Joe that after the test he found Kaino and said 'you got me good'.