Tsurumi Ryokuichi Park is not that famous in Osaka. Japanese people that I have met have not heard of it but it seems that the park is a famous hangout for cosplayers.
Last weekend, lots of cosplayers where in the park for a day with Sakura and photo sessions. I asked one of them if it was an event or something but according to them, they were just there on their own, just having fun, dressing up as their favorite anime characters and taking pictures under the sakura tree.
The park is huge and have several theme gardens from around the world and more than enough space for every cosplayers.