If I am reading your picture correctly, the distance from O to N is 16 and the distance from N to M is 24. That means that the length of OM is 16+ 24= 40. The ratio of any side of the larger quad to the corresponding side of the smaller is 40/24 which is NOT 5/4!
Assuming you do mean PO, PO in quadrilateral MOPR corresponds to TN in quadrilateral MNTS which, according to your diagram has a length of 12. PO must be that length times the ratio.