Apple has made older versions of Mac OS available for free download. Find them at AppleCare Support. Look for System_7.5.3_01of19.smi.bin to install System 7.5.3 (you need all 19 parts) and SSW_6.0.8-1.4MB_Disk1of2.sea.bin for an install of System 6 (you need both parts).
You can also use Sys_7.5.5_Update-NetInstall.sea.bi to update your system.
Newer versions of Mac OS have not been made available for free download.