At the right top side there is space for two tool-free 5.25" drives, with easy drive installation. Let me zoom in on the securing mechanism first though.
You will spot two optical drive bays (5.25") to choose from, each features a latch that has an "open" and "lock" position. You simply pull and release the latch. Mounting a 5.25" device could not be any easier - just line up the holes on the optical drive you just inserted into the case and when you hear a click the pin locks have secured the device. Just below the ODD cage you'll notice a 2.5" compartment, we'll look at that later but on the backside of it you can insert SSDs, really convenient.
Here you can mount lengthy power supplies. There's plenty of space for even the longest units. The Obsidian 350D has a removable dust filter in front of the intake fan and underneath the PSU in the bottom rear of the case.