Sure!
First of all, here's the link:
http://www.ouac.on.ca/omsas/pdf/c_omsas_b.pdf
At the very bottom, you'll find boxes with the different schools and the GPA system they use. Find out the number for your school.
At the top, you will see: 1,2,3,4,5,6,7,8,9 (Numeric, Percentages, Alphabets). The column for your school's number is what you will use, and you will use it to locate your grades. Write down all your courses on a piece of paper. Once you find your grade on your coumn, go to the very right (or very left), where it says OMSAS Value and write down the corresponding OMSAS Value for each grade near the course it belongs to (for YorkU - number 9 - an A is 3.8 so for a course that I've gotten an A in I'll write 3.8). Once you have all your grades written down, add them all up and divide by the number of courses. The result is your GPA according to OMSAS. If your having trouble, let me know your school and grades in a private message and I can calculate it for you.
Good luck!