Overnight-Over-Night | Commonly Confused Words

Overnight

/ˌəʊvəˈnaɪt/

Happening during the night or instantly without delay

Jennifer had even packed Anne's overnight bag."Aye, good chance we end up in the Market overnight."The new policy was implemented overnight.

/ˌoʊvəˈnaɪt/

Happening suddenly or in a very short time, usually referring to an event or change that occurs from one evening until the next morning

Overnight is a single word combining over and night.
Use overnight when describing something happening throughout the night or over an entire night.
Over night, as two separate words, is rarely used unless specifically discussing the concept of over relative to the night, like a question about the night sky.