A shortcut can be set for OWL to toggle between normal and overwrite mode. In addition, speech can be enabled to give an audible indication when switching keyboard mode.
The mac OSX is a great operating system, but lacks an overwrite mode. To be honest in most situations OSX does everything fine.
For example when editing guitar tabs, it is far easier to overwrite numbers then to select a number then type in a new numbers, or forward delete then type in the new number.
One of my bass tabs
I working on OWL now and OWL will be available for MAC OSX Mavericks/Yosemite in the very near future on the MAC App Store.