D&R Marine is THE place for any O'DAY specific parts that we need for our boats! Just remember, D&R is just that....... Debbie and Rudy, Rudy is a great source for O'DAY info and has a lot of innovative ways to solve our needs (he has been known to get very creative at finding savings on shipping!) but D&R is a small shop, and VERY busy at times (Spring launching time brings out a lot of orders!) but Rudy does his best to get things out ASAP. If you order from the web-site, it is best to follow-up with a phone call, sometimes that saves confusion and delay.
Finally, as a reference, here is a copy & paste of the specs and Standard Equipment for the 1978 O'DAY 30. Note that the keel was molded as part of the hull and the original centerboard was (as Rudy mentioned) Fiberglass, with a chunk of Lead Ballast inside. The centerboards for the O'DAY 25/26, or the 23 will not fit the 30, the 30 used a much bigger CB, which unfortunately, D&R does not have the mold for. I think an O'DAY 30 owner has posted a sketch of the CB on this forum somewhere? I know he posted it on FaceBook recently.