Japanese oil trading and shipping companies could have bought into Dutch East Indies’ “Royal Dutch Shell”, British Petroleum, or other oil exploration, production, and refining companies operating in Southeast Asia.

In the 1920’s many of those oil fields in the Dutch East Indies were just being discovered and developed, that was a very opportune time to come in with capital and develop expertise (everyone was figuring it out as they went along, refineries were very small and simple, etc. so entry was mostly dependent on cash and credit.)

A national oil company (more of a later approach that now dominates 80% of world oil production) would have been more worrisome to the Dutch and British perhaps (Japan was a solid British ally then.) As the 1930’s progressed with increasing control of the Japanese government, through assassinations, mini coup d’etats, intimidation, etc. by young officers in the Japanese Army, accessing and controlling oil supply this way was apparently too complex of thinking (stupidity and impatience took over the government while wiser, calmer leaders were displaced if not killed.)

Setting up private oil companies, under Sumitomo, Kawasaki, Mitsubishi, Fuji, etc., one of the industrial firms that built lots of heavy equipment, ships, pumps, etc. would have been simpler and is somewhat surprising it didn’t occur!