Are you sure they need a copy of the GC? My mom doesn't have any, I don't think. She's not careful about these things, so she never thought about it. But, the website says this about OCI:
7. Please provide one:
Two Colour legible photocopies (original may be asked for, if required) of Green Card
or
Two Colour legible photocopies (original may be asked for, if required) of EAD
or
Original Indian passport on which first time entered USA
or
Original I-130 and supporting documents provided at the time of obtaining immigrant visa to enter the USA (see
www.uscis.gov for details of obtaining)
or
If applicant is US/ Foreign born, pl. provide two colour photocopies of the birth certificate (In this case, pl. provide documents as per OPTION B at S. No. 8 below)
So, we can choose one, right?