MACINTOSH OS

The Quadra 605 requires at least Macintosh OS System 7.1
System 7.1 through System 7.5 require System Enabler 065. Download it here
Macintosh OS System 8.1 is the latest version supported without a PowerPC upgrade
Macintosh OS System 8.5 through 9.1 supported with a PowerPC upgrade

MACINTOSH OS DOWNLOADS

Mac OS System 7.1 floppy disks – original System Software 7.1 floppy disks that shipped with the Macintosh Quadra 605 (.dsk)
Mac OS System 7.6.1 CD – bootable retail CD image (.iso)
Mac OS System 8.1 CD – bootable retail CD image (.iso)
Apple Legacy Software Recovery CD – bootable CD image (.iso) with every Mac OS from 1.0 through 8.1
Basilisk II Emulator for macOS – fully configured 68k emulator with Quadra ROM and hard drive image with System 8.1 installed
Basilisk II Emulator for Windows – fully configured 68k emulator with Quadra ROM and hard drive image with System 8.1. installed