mick1052..... O/S = operating sysem ....for example Win 95 , Win 98 , Win 98SE , Win 2000 , Win ME , Win XP .
The issue with the monitor getting progressively darker......I would assume this has been going on for quite some time ......... Given the age of the monitor ...it's likely it may have seen better days........
It may be time for an new one .
dl65