set status_overwrite [<text>]
This variable sets the string displayed in the status bar when the client is in overwrite mode in the input line. If the client is not in overwrite mode (i.e. is in insert mode), nothing is displayed.
This string is represented as %O in the STATUS_FORMAT string. The expando is treated literally inside STATUS_OVERWRITE.
Customizing the string indicating the client is in overwrite mode:
/set status_overwrite (Overwrite)