United Kingdom North Sea oils sourced from the Kimmeridge Clay Formation contain unradiogenic (∼0.17 to ∼0.48) and radiogenic (∼1.04 to ∼3.34) 187Os/188Os values. The unradiogenic 187Os/188Os values are spatially associated with the main basin-bounding faults of the Viking Graben and East Shetland Basin. In contrast, the radiogenic 187Os/188Os values are associated with North Sea basins located farther away from the basin boundary faults. We suggest that crustal thinning and strain localization within the Viking Graben and East Shetland Basin are sufficient to have allowed basin-bounding faults to propagate to sufficient depth to act as conduits for mantle-derived fluids to interact with oil. This hypothesis is supported by previous geochemical data for North Sea oil fields. Furthermore, we demonstrate the use of Os isotopes as an important tracker of crustal-scale fluid dynamics and petroleum migration pathways in extensional basins.