What's the difference between colloquial and oral English?

Solution 1:

Alternatively, for the linguists out there: oral refers communication medium whereas colloquial refers to register.

Considered this way, the concepts are not mutually exclusive: would you consider a speech (i.e., oral communication) given by Queen Elizabeth II colloquial? Surely not! Given by President Barack Obama? Quite possibly…

For another example: colloquial is in opposition to formal (among others), whereas oral is in opposition to written, signed, etc.

Solution 2:

"oral English" is any spoken English. "colloquial English" is informal, and may include language that might be unacceptable for printing.

Solution 3:

Colloquial comes from the word conversation, specifically, according to the OED

 1. Of or pertaining to colloquy; conversational.

Perhaps a better way to phrase your question is - What is the difference between oral and conversational English.

Conversational English is typically oral, but oral English is not necessarily conversational.