OS 10 Onyx is not an operating system that actually exists.
Go to the Apple Menu (top left of the screen) and click on it. If there is a System Profiler or an About this Mac/Computer, click it.
It will tell you what version of OS X you are using. On a G3 iMac I would not expect it to be above 10.3.9
What does the desktop look like? Does it have the distorted image and text that does not appear unless highlighted?
P
I have an iMac G3, 400 mHz and 256 memory. it is and has run OS 10 Onyx for the last year. Recently when a progrma opens the image is distorted, text does not appear unless highlighted and the tool bar icons are unintelligible. I don't know much about computers. Does anyone have a suggestion for fixing this problem.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic