The coach claimed that Mitchell, who failed to crack the Top 40 with her single 'Run to You', isn't an exciting artist.
"She wasn't on my team so I can't feel that much disappointment," O'Donoghue told Gigwise. "She won the show, you know, in the final she was technically the best singer. Was she technically the best artist? No. But that's just my opinion.
"She was definitely the best singer on the night, but it was unfortunate that other people didn't get a look in, ones that would have suited pop music better.
"She's got an amazing voice, but I couldn't see her exciting 80,000 people for an hour and a half. There were people like that on the show, but they got voted off, and it's quite hard."
Mitchell, who was mentored by Sir Tom Jones, defended her single's chart performance, saying it wasn't given a chance.
Watch Leanne Mitchell's video for 'Run to You' below: