In △APB and △ECB,
∠ABP=∠EBC (Common angle)
∠PAB=∠CEB (Corresponding angle of parallel lines)
∠BPA=∠ECB (third angle)
Thus, △APB∼△ECB by AAA
ABEB=BPBC
2EBEB=PBBC (E is the mid point of AB)
BP=2BC
BC+CP=2BC
CP=BC or PC=AD (Since BC = AD)
Now, In △s AOD and COP,
∠AOD=∠COP (Vertically opposite angles)
AD=PC (Proved above)
∠ADO=∠PCO (Alternate angles for parallel lines BC and AD)
Thus, △AOD≅△POC (SAS rule)
Hence, OA=OP
or O is the mid point of AP.