The igneous and metamorphic rocks which underlie the sedimentary section of an area (colloquially referred to as “basement” rocks) exert substantial influence on the overlying younger rocks. Large structural features are rooted in basement rocks, and additionally the basement usually contains the only available information regarding the pre-sedimentary geological history. Secondary alteration processes in basement rocks can significantly change their mineralogy and mechanical properties. Understanding of geological history and regional structures within a given area therefore requires study of its basement rocks. This dissertation utilizes petrographic, geochemical, structural, and magnetic approaches to study the basement rocks of Oklahoma. New petrographic observations of the Mesoproterozoic Osage Microgranite in northeastern Oklahoma provide evidence of deformation and secondary alteration and suggest a possible extrusive origin for this unit. Geochemical data additionally suggest a subduction-related origin. Study of several cores in northeastern Oklahoma shows that fracturing and hydrothermal alteration are pervasive in the area’s basement rocks, in contrast to common assumptions of intact granite used in modeling studies of induced/triggered seismicity. Magnetic susceptibility measurements of some altered basement rocks and overlying clastic sediments show highly unusual properties, including a strongly negative relationship between susceptibility and the strength of the measuring field. Detailed study shows this is likely due to an unknown magnetic mineral which saturates at very low fields and has a Curie temperature near 85 °C. Paleomagnetic study of the ~1370 Ma Spavinaw Granite yields evidence of remagnetization in the early Cambrian and in the middle Paleozoic. Magnetic fabric data suggests it is either strongly tilted relative to its original emplacement or was emplaced as a dike rather than a horizontal sill. Paleomagnetic data from intermediate and mafic rocks associated with the early Cambrian Glen Mountains Layered Complex in southwestern Oklahoma provide a new constraint on the paleogeography of Laurentia, with implications for various hypotheses relating to early Cambrian geomagnetic behavior. Additionally, some sites show evidence of partial to complete remagnetization in the middle and late Paleozoic.