Im in my third year of the medical radiation science program and my gpa is a lot higher than i expected it to be. So my question is if I can come back to take additional courses which I never took during undergrad (orgo chem, physics, calc) so I can apply to med school? It would just be for 1 year.

You can, but you'll be considered a continuing student which means no OSAP.

Just do a bit of research first into which courses you want to take and for which schools your GPA would be competitive at. Focus on the MCAT and you may be able to teach yourself more than you think. An FYI - nowhere in ontario requires any of those courses with the exception of Ottawa where you need both chem and ochem or biochem.

Wait, you mean taking a year load of courses after you graduate?
That's considered a continuing study and I'm pretty sure most med school don't look at non-degree-leading GPA.
If you're trying to do an extra year after graduation just to fulfill some of the prerequisite courses, then the best way is to contact some med schools directly and ask them.

It's pretty easy to break the system. Just leave one course you require for your degree within the extra year you're taking, and then take others to make yourself a full time student. That's what I did (inadvertently) and I managed to get full OSAP.