Ceramic tile needs backerboard for a substrate---that is the cement composite stuff. The application of thinset on plywood/OSB causes swelling due to the moisture and can cause loose bonding.

Thanks for the good input handymac. I agree with your recommendation.

To be more specific the 'plywood' is actually for under the whirlpool tub (ie. the box that the tub sits in which will be finished with ceramic tile).

There will be an opening on the top of framed-in 'box' which the tub will be sit in - so I want to put tile on this top surface. I could cover the top (maybe even over the plywood - how would I join the cement board to the plywoood? screw it?) with cement composite board but can I just (thin set) adher the tiles to the plywood?

- or will this be a problem?

I install backerboard on a bed of thinset and use screws to secure. Then another layer of thinset for the tiles.