I'm not finding any reference to such a light for 2005 Mustangs, in the Owner's Manual or Alldata's Technician's Reference. What does this light look like?
If you do have an oil life monitor, an OBDII scanner is not needed to reset it, and I doubt it even could, or give you any other pertinent information for that matter.
Every oil life monitor I've ever heard of has a reset procedure outlined in the Owner's Manual, as well as an explanation of what the light means. I am finding no such information there, or in the Alldata Technicians reference.
You can download the former here;
if you'd like to check for yourself, and maybe that will also tell you what that light is.