The goal of this thesis is to estimate the amount of extension and displacement across faults using geometrical constraints. The mapped area is dominated by middle Ordovician to lower Silurian sediments folded during the development of the Scandinavian Caledonides, followed by the development of the Oslo Rift in late Carboniferous. This led to the formation of extensional structures and magmatic intrusions in the inner Oslofjord. The recorded fault planes and dikes throughout the study area show a NNW-SSE trending orientation. Extension in the study area has been calculated using different trigonometrical methods. Results from calculations have been used to estimate the stretching factor, correlating with previous work within the Oslo Graben. From observations in field and data processing, different hypothesis has been tested to try and interpret how the Oslo Rift developed based on the measured dikes and subvertical faults in the mapped area.