Computational condensed-matter physics, as a branch of modern solid-state physics, comprises the field of ab-initio calculations. Nowadays, challenging parameter-free studies can be carried out that deal with the many-body aspects due to the involved electron-electron interaction. Theoretical-spectroscopy techniques provide insight into electronic excitations, paving the way towards computer-aided materials design, e.g., for photovoltaics. In this thesis the transparent conductive oxides MgO, ZnO, and CdO are investigated; they are important materials for transparent-oxide electronics. Initially, a hybrid functional is used to model exchange and correlation. Subsequently, quasiparticle energies are calculated using Hedin's GW approximation of the electronic self energy. This leads to band structures, densities of states, spin-orbit splittings, effective band masses, and natural band discontinuities. Solving a Bethe-Salpeter equation for the optical polarization function yields the complex dielectric function including excitonic and local-field effects. All underlying atomic geometries result from density-functional theory based on local or semi-local approximations to exchange and correlation. Furthermore, this thesis deals with imperfections that affect the electronic and optical properties. The influence of uniaxial and biaxial strain on the band structure of ZnO is explored. Iso- and heterostructural alloys are investigated, taking the sample-preparation conditions into account via a cluster-expansion method. Exciton binding energies for the defect-related optical-absorption peaks are calculated for the oxygen vacancy in MgO. Effects due to free electrons in heavily doped ZnO are incorporated into the description of the optical properties. This allows to study the Mahan exciton as well as the inter-conduction-band absorption in this system and explains why a Mott transition of the exciton is hard to observe. The agreement with experimental results is reassuring.