You can lay ceramic directly but only under certain conditions.
You need 1 & 1/8" minimum, no sheet thinner than 3/8".
The ply has to be CC plugged grade or better. I can never find that by me so I get BC grade. No CDX is allowed, its got a sheathing rating only, not and underlayment rating, and the "D" layers have way to many voids which can allow for compression under weight, thus causing the tile to crack.
You need to use a modified thinset that says on the bag it meets or exceeds ANSI 118.11 standards. This is a case where you get what you pay for so go for the most expensive thinset you can find that meets the standard. I buy one made by Tec, called Super Flex, runs about $40 a bag.
The ply sheets need to be gapped 1/8" at the seams and gapped 1/4" from any wood studding around the perimeter of the room to allow for seasonal movement. Fill the 1/8" gap between sheets with a cheap latex caulk and let dry before you spread the thinset.
The boards need to be screwed down every 8" along the perimeter and through the body of the board.
Get the video called "Trowel and Error" from the National Tile Contractors Association www.tile-assn.com The best $10 and 10 minutes you will spend on this project. It will show you the proper way to spread the thinset and place the tile in the thinset to ensure a complete bond