Originally Posted by
Bandicoot
[...] since about 2015 the need to carry the old passport was eliminated. I have personally traveled many times to India with a new passport and the OCI booklet, and without the old passport, and never been questioned about it by any airline staff or immigration officer.
Updating this thread so it has current information for anyone reading it -- the rules have changed again and as of January 2020 the text in TIMATIC now
requires anyone over 50 to carry both the old (used for OCI issue) and new passports in case the passport number in the OCI does not match the number of the new passport. So no more can one skip carrying the old passport.
The text (from the Korean Air visa info page) is:
Passengers with an Overseas Citizen of India (OCI) card younger than 20 years must have both passports when the passport number in the OCI card does not match the one in the valid passport.
Passengers with an Overseas Citizen of India (OCI) card 50 years or older must have both passports when the passport number in the OCI card does not match the one in the valid passport.