You can but it is not recommended. It all depends on the substructure anyway. You need to get a good bond between the substrate and the tile or else the tile will pop up and moisture will get in between the tile and substrate. A couple of areas that are particularly susesptible to this is near the sink and over the dishwasher. There are a bunch of different bonding agents that go on like paint at tile stores. Talk to someone working there and tell them what you are doing they should be able to point you in the right direction. Replacing the plywood with cement backer board or other products meant to be used as tilebacker would be the best solution.