Incorrect spelling, explanation: on top is a phrase made from preposition and a noun, and many others are created in the same way – ontop feels nice to say like it’s one word, but it’s important to remember that it’s still two separate words with separate meanings.

Correct spelling, explanation: on top is a phrase made out of two components: a preposition on, and a noun top. The phrase is analogous to other similar phrases describing positioning: on the bottom, on the left, etc., so while on top is the easiest to pronounce like one word, it’s still two separate words creating one phrase.

Definition of on top:
prepositional phrase
1. To be in the highest position of the tallest structure.
It was a beautiful cake with a big “Happy Birthday” written in chocolate on top.
2. To be in a powerful, dominant position, above others
He studied really well so he was always on top of the class.
3. To be leading in a sports game.
Our team was on top in the first half, but we lost in the last seconds.

Idioms and phrases with on top:
To be on top of something – to be in control, to know exactly what to do about something
I used to be scared of taxes, but now I’m on top of all my finances!
To be on top of the world – to be very happy, joyous (usually: to fall on top of the world)
She felt on top of the world when he finally proposed.