Before we begin, English is not my first language so my terms could be a bit wrong.
Considering I'm planning to install 1/4" concrete backer-board over the sub-floor before tiling it again, is OSB okay as a sub-floor or does it have to be plywood?
I'm in the process of remodeling our upstairs bathroom in a condo unit. What is there now is a 3/4" OSB sub-floor (over which tile was directly installed). Before we bought the condo, the toilet leaked and got shaky (and unusable, as it wanted to go through the floor; fortunately, there was a joist just underneath to support it); it's dry now but the OSB sub-floor around where the toilet was is completely rotten and has to be replaced.
I believe the original contractor who built the house did a sloppy job in there, 1st by installing OSB in the bathroom instead of using plywood and 2nd not putting any backer-board before installing the tiles and 3rd by the way the spread the thin-set under the tiles (the "curvy/old" way).