You can mail just one money order. I have applied for my SC + OCI last month, and provided just one money order.
Before you get the process going, read this first: indiavisa.travisaoutsourcing.com/oci/display-requirements
Complete the OCI Online Application Form on MHA website.
After you have completed the OCI Online Application, then complete the online Order form on Travisa's website. It will provide a detailed checklist and instructions, along with the amount for which you need to provide the money order for.