Is did a preposition

According to Merriam-Webster, “overall” is an adverb that means generally or as a whole. As a noun, overalls are pants with a bib and houlder straps, or loose, protective trousers worn over your normal clothes.

As an adjective, “overall” means general, including everything, or viewed as a whole.

Examples:
Overall, I think the drilling went well. (adverb)
Put on overalls if you are going to work with acids.

Is for a preposition

(noun)
Unfortunately, the acid stimulation was an overall failure. (adjective)

“Over all” is an expression that contains a preposition (over) and a pronoun (all). It means “above the whole thing” or “on top of everything.”

Examples:
Make sure you put tarps over all the equipment to keep the rain off.
“I am Yertle the Turtle, oh marvelous me, for I am king over all that I see.” – Dr.

Seuss

If in doubt, substitute the word “general” or “generally” for “overall” in the sentence, and if it fits, use the single word. If it doesn’t fit, use “over all.”

Exa

is contain a preposition

is all over a preposition