OEM+ stand for "Original Equipment Manufacturer +".
This term originally comes from the automotive world to describe the customization of a car in a subtle way, (mainly) using parts and accessories from the same manufacturer or the same group (Using Audi parts on a Volkswagen for example) . More generally it means customize a car very discreetly, with a close look to the original, playing on the details. In contrast of tuning and its extravagance.

— abbreviation for
1. OEM Audio Plus

OEM Audio Plus designs the equivalent of a tailor/custom suit for your vehicle's sound system. Delivering some some seriously high quality audio.