Question

what is the solute and solvent in a solution made from 15 mL of oxygen and 5 mL of helium?

Answer 1

Most would label helium as the solute, but you are dealing with miscible substances. Look up the term miscible.

The use of the terms solute and solvent in miscible combination is not useful, generally. I am not a fan of using these terms in miscible gaseous solutions

Answer 2

Nice