It seems to me that OL means it is a like a discount or a promo you encash on a car, as far as some of the European results I've seen on google. Maybe a specific dealer car, generally not sold to the general public.. This is the best I came up with.
"Notes on using OL codes:
Dealer participation varies. Non-participating dealers may apply an equivalent discount when you mention the OL code. It’s subject to dealer discretion and we aren’t sure how or when OL codes are being generated
OL codes can be used on new vehicles. Some dealers may not accept them on loaners
Some dealers will apply $500 or $1,000 as dealer discount when you mention the OL code
I’m hearing OL codes in most cases can’t be stacked with USAA or Fleet, but this isn’t guaranteed
In some cases, dealers will only offer $500 of the $1,000 (we’re hearing that $500 of the code is a BMW NA rebate and the other $500 comes as a dealer discount). Per @BMW_Dave, OL codes must be provided so the dealer can look them up in their system to confirm validity and apply the $500 BMW NA contribution."