Celtic qualified for the knockout stages against the odds after beating Barcelona and doing the double over Spartak Moscow.
O'Neill led Celtic to the UEFA Cup final but could not make the breakthrough to the Champions League knockout stages.
And he believes Lennon, his skipper at Leicester and Parkhead, deserves enormous credit for doing so in tougher financial times for the SPL, which lost Rangers in the summer.
The Sunderland manager said: "That was a fantastic achievement. With the SPL maybe not as strong as it was a couple of years ago, their performances in Europe are even more remarkable.
"I thought qualifying for the group stages was still a great achievement, they had to knock out the Swedish champions (Helsingborg).
"To get through that group is just remarkable, it's absolutely fantastic. Well done to the club and well done to Neil Lennon."