Can you use on top of as a synonym beside?

No, "on top of" and "beside" are not synonyms. They have different meanings and cannot be used interchangeably.

"On top of" means:

* Above: something is situated directly above something else.

* In addition to: something is added to something else.

"Beside" means:

* Next to: something is situated alongside something else.

Here's an example to illustrate the difference:

* "On top of": "The book is on top of the table."

* "Beside": "The dog is sitting beside the chair."

You can use "in addition to" as a synonym for the second meaning of "on top of".

For example:

* "On top of the cake, there was a cherry."

* "In addition to the cake, there was a cherry."