It's not a button or a key on the keyboard, it's in a document. Open a word document, and at the bottom there are several options. OVR is one of them. Click on it, it toggles. I'm not in front of my Mac so I can't take a screenshot.
In the bottom right of this picture you can see where they start:http://www.macdevcenter.com/mac/2002/04 ... oc_mac.jpg
Too big for me to put in my message... but it's the very bottom left next to the TRK button that you are looking for.
I'm new to this forum but just want to say a HUGE THANK YOU to Mr Colby - I got stuck really badly and did a Google search - most people asked this question because they wanted to insert something, ie. Command + V (which is paste somethign that's been copied). In my case, I had hit something and as I'm typing, the letters on the right that appear on my page were being "eaten" by my new typing. With a PC keyboard, I'd just press the "Insert" button. I tried all the buttons on the MaC keyboard - and found a post where someone said to use the "Help" button on the keyboard - well,my MAC keyboard does not have a "Help" button, very helpfully!
Some 13 years later, your post (and the person who posted the original query) has more than done it's job to help others. Thank God for that.