very easy. pay $50 to the univ. regis if meet the requirement for full-time. no need to OUAC anymore.
univ. regis is different from department regis. keep publich adm if it is already in adm school. you can change your major without fee before the deadline in the department.
if you want to change the department/school to another, you must reapply, but no need to OUAC.
And you have to meet the ADM's requirements. You better to check the deadline(!) both university and school.
if you have missed the application deadline of university,there is another alternative that in the social science school, select an option including as many as ADM courses and then change school in winter semester.