Abby_777,
It's much of a muchness really. An OD means anyone can turn up and be considered whereas an AD means your online application will be screened before you are invited to attend the AD. ADs do tend to be smaller groups but their size can vary.
You can apply for ODs and be invited to them as well however the competition can be fierce - as in 350 other applicants may turn up! If you choose to attend an OD as opposed to the invited AD you may need to make your venue selection wisely. Choose a location in a smaller city that will probably have fewer people turn out. When I went to my OD (in the west of my country) last year there were 83 applicants and 5 made it to FI whereas 2 weeks before (in the east) there was 350 applicants and 5 selected...
F