(HL) Axiom of hypotenuse and right angle: Two right angle triangles with hypotenuse and right angle are congruent.

So the front should be:

OP=PO (hypotenuse)

OM = On (right angle side)

Or use: ① Angular axiom: two triangles with equal included angles are congruent (can be abbreviated as "angular" or "SAS"). But this problem obviously uses HL theorem, not SAS theorem. Your method is not SAS, because the diagonal you use is not the included angle.

You can ask if you don't understand anything.