I have used a platypus in the past, they were OKish but not as well made as the Osprey bladders which are designed to fit the pack. Moreover, their after sales service is excellent. In my case, when the lid seal developed some black spots after several years of use, they replaced it free of charge.
If your Osprey pack is like mine, there is a small strap located just inside the H2O pocket that will fit through the slot in the top of the plastic frame of the Osprey bladder. This holds the bladder in the ideal position so that it does not slip down too far and keeps the pipe at the ideal length. The bite valve has to be turned through 90 degrees to open it which prevent leaks when not in use and has a magnetic attachment to the rucksack strap - quick and faff free. Moreover, the valve comes apart easily for cleaning.