You can calculate the OROP arrears in the following way:
1. Find the Basic pension as per OROP chart table. say for Example; Hav and equal Ranks Sgt Group X QS:15 yrs.
As per Table no: 7 of OROP Tables  8585 
As per Cir 501,table No.17  6374

Difference per month  2211 
Now the Effective Date is From 01072014
Arrears from 172014 to 31012016 will beDifference X a factor of 40.53 , 2211 X 40.53 = 89612/
SO 89612 IS THE ARREARS
This will be paid in 4 equal half yearly installments.
For family pensioners, in one installment.
This the simple way of calculation.
Sourcehttp://exweltrust.blogspot.in/