In the diagram below, P(12;-5) is a point in a Cartesian plane with origin O(0 ; 0). The reflex angle that is formed by OP with the positive x-axis is $\theta$ .

Determine, without using a calculator, the value of each of the following:
- The length of OP
- $5\cot{\theta}-13\cos{\theta}$
- $\text{cosec} ^2{\theta}-1$