The goal by the Tottenham loanee rounded off a good week for the home side who opened the scoring through Johnnie Jackson before Luke Varney equalised for Leeds with the game heading for a draw before Obika's late intervention.
With Charlton earning seven points in a week, Powell was full of praise for his side after their first stoppage-time goal this season.
"I'm really pleased with the whole week, ecstatic with the performance - we didn't deserve to be at 1-1," he said.
"It has been a pivotal week, to get seven points and a real gritty, dogged point at Brighton. We have to do these things to survive in the Championship. And there's no better way of winning than in the last minute.
"We knew we had that tag of least home wins, but that's gone now. Our away form has kept us going, and during the international break I thought that we had four home games we could win.
"To beat Leeds was fully deserved, we have Wolves and Bristol City and we can keep carrying on. Any point in the Championship is massive - I don't want our players relaxing. I've set them no targets."
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