Integrated energy modeling is increasingly important for planning industrial energy transitions. This study examines Serbia's oil and gas sector through NIS j.s.c. Novi Sad, analyzing energy flows, material balances, and strategic pathways. An Integrated Energy Model was developed in Low Emissions Analysis Platform (LEAP) for 2023 using a bottom-up approach, with projections to 2030 and 2050. Three scenarios were considered: Baseline, Energy Efficiency, and Green Agenda (Renewables combined with Energy Efficiency). The Baseline scenario forecasts total energy use rising by about 4% by 2030 and 2% by 2050. The Energy Efficiency scenario reduces consumption by 6%, while the Green Agenda achieves a 32% decline through full renewable integration and demand-side optimization. Oil, gas, and derivatives remain dominant, though production drops slightly in the Baseline and up to 50% under transition scenarios. Processing grows moderately in the Baseline but falls significantly in alternative paths. Electricity generation expands in the Baseline, while transition scenarios reduce demand and shift toward renewable sources. Greenhouse gas emissions remain almost flat in Baseline but fall by 32% in Energy Efficiency and 73% in the Green Agenda by 2050. Blue hydrogen from the Pancevo refinery is included, while green hydrogen emerges as an additional lever. Findings show that quantified, operational measures, supported institutionally, are essential for oil companies to move beyond declarative strategies and secure an optimal long-term energy mix.