OCI Cardholders who stay in India for over 182 days in the 12 months immediately preceding the date of application for Aadhaar and have an Indian address can also enroll for an Aadhaar Card in India. But others shouldn't be doing so. That said, there are lots of Indian-residential-property-owning OCIs in the US and Europe who have gotten Aadhaar despite not formally meeting such kind of 182-days-in-12-months Indian residency qualification at any points since Aadhaar came into existence.

OCIs shouldn't need Aadhaar to be able to sign up for or retain mobile service in India, but perhaps there are still retail outlets around that aren't familiar with how the Indian Department of Telecom has formally approved an exemption/alternative for OCIs/PIOs and NRIs a few years back.