Make sure the mast is mounted correctly and is not leaning to one side. Take careful measurements with a steel tapemeasure to verify.
Other, ... take a good look at the location of the batteries to be sure that they balance to the centerline.
You may want to join the O''Day email list (excuse the pun) here on SailNet. Ask the question there. You''ll probably get connected with more O''Day owners there. Just click on Join E-mail Lists which is over on the left side of the computer screen.
How far is it listing? How long has it been doing this? Can not imagine it would be related to batteries. Is the keel ok? Has anything been added onto the boat recently? The previous question about the mast is a good one? Also is it listing visibly, or according to an inclinometer (i.e. is the inclinometer installed correctly???). Good luck.