Where did Octan come from? Why was it chosen?
Octan, is simply Octane in Danish, which is a chemical in and associated with gasoline making it a sensible name for a fictional petroleum/energy company.

The LEGO Group (under pressure by a Greenpeace campaign) stopped the previous corporate partnerships, which had featured real world oil companies logos in LEGO sets, to distance themselves from the environmental protests associated with those companies.

All this can be found here:
https://en.wikipedia.org/wiki/Octan