Is wood ok to use as a sub floor when laying tile?

Wood is fine for your SUB-floor, but not as the substrate to which you intend to attach the tile. You still need to install a crack isolation membrane or tile backerboard on top of the wood subflooring. It's because wood has too much 'give' and will cause your tiles to track and possibly pop loose. Grout lines will crack and eventually chip out.

I followed instructions exactly for installing tile to my plywood subfloor - I used a polymer-modified thinset mortar designed to work with wood, and my floor joists were 16" on centers. The only difference is that I had a double thickness of subfloor (1-1/2") instead of the recommended minimum of 3/4". Ten years later, I have several tiles which sound 'hollow' (loosened from the subfloor) and several others are cracked. These are high-density porcelain floor tiles. I'll not do that again.