First off: they don't allow chalk because they are too lazy to clean the holds... This means the holds are polished anyways by people's sweating hands. You're supposed to use "Eco-Balls" which I understand for outdoor, but indoor?!?
Whatever, good selection of bouldering lines and they have top-rope. Can't remember if they have lead climbing...
Check the website for opening hours:
750m of climbing wall...
Opposite is Vienna's best outdoor good store called Bergsport Schwanda, but the OTK has a good selection of climbing gear as well.