He completed an evaluation day with GRM last December, and one with Erebus Motorsport in June.
O'Keeffe won't be the only Super2 driver in the 12 Hour field, with Triple Eight recruit Brenton Grove set to steer a Class B Porsche.
A total of 18 drivers who contested last year's Supercheap Auto Bathurst 1000 – either as primary or co-drivers – are also in the 41-car entry list.
That is headlined by Triple Eight's Mercedes for Jamie Whincup, Shane van Gisbergen and Craig Lowndes, an Erebus Mercedes with David Reynolds and Luke Youlden and Nick Percat and Tim Slade sharing a Ferrari.
Chaz Mostert and Garth Tander are on factory duties for BMW and Audi respectively, with Lee Holdsworth and Todd Hazelwood also in Audis.
Practice for the 12 Hour starts on Friday at Mount Panorama, with 7mate to broadcast Sunday's race live.