High demand and a significant increase in the price of crude oil have triggered the oil and gas industry to produce more oil and gas from their assets. Optimizing mature fields is one of the efforts that can be applied to increase production. "RBT" field is located in South Sumatera, Indonesia, and was discovered in October 1972. It has produced from multilayer "PLB" sand reservoir since 1974. Originally this field was produced as an oil field, with a peak rate of up to 1630 BOPD, declining to 300 BOPD 9 years later. Thereafter the field was converted into a gas field producer with small condensate and is still producing now. Based on the above, it is very challenging to look for by-passed oil in "PLB" sands reservoir, since the production test data and history of this field are very limited, so the field can produce more oil and maximize the recovery factor. This paper explains the steps that need to be taken to analyze by-passed oil in a multilayer reservoir, including: production data test review, surveillance data analysis, historical completion review, sand layer re-correlation, estimating update reservoir contact and updating geological map. After all of these steps have been completed, work over and well service jobs will be used to validate the analysis and evaluation, allowing further development strategy to be prepared. This method has been proven in "RBT" field, which is currently producing oil at around 200 BOPD, 2.5 MMSCFD and also found by-passed gas potential in "PLB" sand, in addition to by-passed oil. This production will increase gradually later through the addition of more work over or well service jobs. Introduction "RBT" field is one of mature field that is operated by PT Medco E&P Indonesia. It is located in South Sumatera, Indonesia, and was discovered in October 1972. This field was originaly developed as oil field since 1974 and has produced from a multilayer reservoir named "PLB" sand, which has 6 (six) main productive layer zones and several other thin layers. The peak oil production was 1630 BOPD and then was declining to 300 BOPD in 9 years production time. In 1990 this field was converted into a gas field producer with small condensate and is still producing now. As mature field reactivation and optimization is becoming an important aspect for increasing oil production at this time, effort to find bypassed oil in "RBT" Field was started in mid of 2011, started with initial field review, data rebuild and update, workover candidate selection and then followed by re-activation and workover campaign in 2012. Field review, rebuild & update data The initial effort to find by-passed oil potency in "RBT" Field were examining cumulative production allocation and analyzing historical production data from each zone and sand layer. These steps also became the main challenge as there were lack of detailed historical oil production and allocation especially in commingle zone.