If your applying for a tuition fee loan for OU study then you need to apply for a part time tuition fee loan.
All OU study is part time from the perspective of student finance and a different loan to the one for campus uni study.
I think the provider reference number is needed for the 24+ loan.
24+ is the wrong loan for OU study because that's the loan for further education or vocational study.
OU study is higher education.
I hope this helps but let me know if you have any questions, or of course call the OU or Student Finance England.