Fletcher proved the difference at St Mary's, sweeping home a ball from the left to earn three points for Sunderland.
"Fletcher has been fantastic for us, really fantastic," O'Neill said.
"It looks like Sessegnon has miskicked it, thank God. He should score more goals but I am glad he left it or 'passed' it to Fletcher.
"His contribution to the club this season has been immense. It is not just that he scores a goal or two for us, but they have been big goals."