We visited the castle and park on a Saturday during July and it was packed. The park was full of runners and kids playing baseball. The area directly around the castle was full of tourists and the castle itself was packed too. The castle is beautiful and has a relatively interesting museum inside although it had limited English explanations. I would definitely recommend a visit if in Osaka, however, would avoid the weekends if at all possible.
If you own or manage Osaka Castle Park, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.