I don't want to spread misinformation as my knowledge is a few years out of date, but used to be OO was not Cat III rated, while QX absolutely is. That can make quite a difference when the weather isn't so nice...
well well…that might explain why my flight diverted to LAX due to a supposedly low ceiling and “all” flights being diverted…even though I saw no such thing affecting SAN after we landed. Cost me a secured upgrade too.