O Original oil in place calculation and production optimization of reservoir need to do for remaining reserve analyze from reservoir itself. Original oil in place equation which used in “S” Field is volumetric method. Productionperformance prediction analyze of oil reservoir is using Decline Curve Analysis. Volumetric method that used for original oil in place determining in S Field has result about 18199 MSTB. In Decline Curve Analysis for each well andfield choosing 2 production trend that used for trend comparing which described production decline in mentioned well. Trend period analysis which chosen was did by Trial Error method and X2 Chis-Square Test and using OFM software as it comparison for determining exponent decline value (b), rate of decline (Di), andDecline Type Curve. Based on per well decline analysis of “S” Field, retrieved type ofexponential decline (b=0), with each well results are, well ST-01=0,147 /month; ST-02=0,047 /month; ST-03=0,150 /month; ST-06=0,200 /month; and ST- 08=0,110 /month. Meanwhile based on field decline analysis, retrieved b=0 with Di=0,041 /month. EUR of “S” Field till qlimitis 8558,75 MSTB which has produced along 63months (5 years 3 months) from May 2014 till Jully 2019. Recovery Factor of “S” Field is 47.02 %. So, remaining reserve of “S” Field isabout 280.915 MSTB.