Oil and solar don't really compete that much. Oil - even at $40 a barrel - is substantively more expensive than pretty much everything else for generating electricity and is rarely used for anything beyond small scale generation.

Oil is valuable because of gasoline/diesel which about 3/4 of the barrel goes to. It's all about the ease of transportation that ICE/gasoline/diesel offer that no other fuel can really compete on.

The more relevant factor is the price of batteries which might make solar and other electricity sources viable competitors to oil.