When users search for a "verified file system" in the context of downloading or setting up OPL, they are usually referring to the specific formatting requirements necessary for the PS2 to read the storage device. The file system you choose depends entirely on your connection method.
OPL is the software application that manages game ISOs. It acts as a bridge between the PS2 hardware and your storage device (USB drive, internal HDD, or SMB network share).
For , OPL uses a hidden partition named +OPL . For USB / SMB , it uses folders on the root of the device.