I run MacOS X 10.3.9 on it. Nothing higher. Because of one program... iPiano (Cider Software), which doesn't run properly in Tiger (10.4) or run at all in Panther (10.5) and the dev has disappeared completely and the software is abandoned. I use my G4 QS for only one thing... making MIDIs. I don't go online with it. It would be awesome if I could successfully transport all my iPiano MIDIs (the native format is a type of .XML; I can export them to General MIDI, Quicktime MIDIs, or .AIFF) to GarageBand (which I use on my M1 Mac Mini) and have them sound just as good (or better) than they do in iPiano, but I don't think that's likely. I just upgraded the rotational IDE hard drive to a SATA SSD (thank you OWC!). Not sure it helped, boot-wise (it doesn't feel like it boots much faster), but it's definitely faster... via xbench. I did it to insure the drive wouldn't die and I lose all my MIDIs (I've been making them since 2005 or earlier).