Your only option to use Octane Render on this machine would be to buy & external module connected via Thunderbolt. Simple piece for single GPU would cost couple hundred dollars & more "matured" with possibility to connect 3-4 cards would stand 1000-2000$ depending on the model & company (that's enclosure only without any cards) - that's a sad reality of those small bins.MAC + eGPU thread
is a place You can start searching for answers - Guy done good work trying to explain & test out some I'd say pretty cheap option to add monstrous amount of power to macs whether that's MacBook, iMac or larger bin..-well, at least I guess things are more or less the same on them (not so much of a mac user myself).
or, build a node..& connect remotely - not like work on MAC & have a render node, but work directly on the render node. If You can live with that, I mean using windows baed PC, that might be best option in terms of value, expandability & overall flexibility =) but I guess, that's not for everyOne..