1. burn the midnight oil - Verb
2. burn the midnight oil - Phrase
burn the midnight oil (third-person singular simple present burns the midnight oil, present participle burning the midnight oil, simple past and past participle burned the midnight oil or burnt the midnight oil)
(idiomatic) To work studiously, especially late into the night.
He was burning the midnight oil to finish his paper.
She was burning the midnight oil to finish her part of the project.