The 31-year-old Munster number eight has been in magnificent form for the Irish over the last year and will hope to impress Lions coach Sir Clive Woodward during the forthcoming Six Nations.
And while O'Sullivan is quick to salute Foley's impressive development, he points out the 57-cap international must continue to look over his shoulder.
"Anthony Foley is an experienced player. He's worked very hard on his fitness and has always been a very good footballer," said the Ireland coach.
"His experience has grown so much that at the moment he's playing better than ever, but a lot of that is because he knows he has to.
"There are guys who are chomping at the bit to take his place and that's a reflection of the quality of the squad."