An oil-gas vapor collection, storage, and recovery system ensures that no air or oxygen leakage into the production tank occurs when gauging and/or emptying a tank. The system provides a constant reservoir-type storage system by utilizing a variable volume gas bag, expanding with a surge of gas from the separator and with minimal water column pressure from the plunger lift system, thereby containing the gas surge, but contracting with gas dissipation thus minimizing the compressor cycling, while accommodating rapid liquid and vapor influxes into the tank thus maintaining constant tank pressure. With expansion, the bag actuates a switch which activates a compressor, which, in turn, compresses the contained gas into the pipeline. Upon bag collapse, a switch is activated to turn off the compressor. Thus, the system provides constant storage tank pressure because the gas bag accommodates and controls variable gas volume.