"I have a pretty good headset on the second row situation. I wish I had the same headset on the back row. Carrying two back rows into the game on the bench on Saturday is a chance to see them playing. Alan (Quinlan) can cover second row if needs be," said O'Sullivan.
"It's about trying to get more access to those areas of the team where I need more information. The back row, that is causing me the greatest headache. Like in the last World Cup, we have an embarrassment of riches in that department. I'm just giving myself one last chance to look at all the options."
O'Sullivan has plenty to ponder when he considers his back three combinations for the tournament, with full-back Geordan Murphy, wingers Brian Carney and Tommy Bowe and centre Gavin Duffy expected to be closely monitored during Saturday's game at Murrayfield.
"Brian and Tommy are both good strike runners and footballers. Tommy has been in the squad for a while now. He's a very capable player. We took a punt on Brian in taking him on the summer tour to Argentina but we what we saw was very positive and we have to give him another opportunity.
"There's nothing in between the two of them so Saturday is very important. It's hard to differentiate between them," he added. "I'm also having another look at Gavin Duffy in the middle of the pitch after he performed well against Argentina. Geordan could do with a game at full-back. He hasn't really featured all year for us and needs to get his confidence back."
To view photos from today's training session at St. Gerard's in Bray, please click here.