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?

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