I am a full-time student with a T2202A. I received OSAP & a grant last year, accompanying T4A slip box 105 In which Box 105 should I report a grant and osap.

You'd put your scholarship and grant in the Box 105 that matches your situation. 

For example, if you have a Full-time scholarship and also have T2202 with months in Box 25, you would use the 1st Box 105.