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In this thesis, oxygen incorporation in M2AlC phases and TiAlN was studied by ab initio calculations and experiments. It was shown that oxygen interstitials are important in both material systems since they are spontaneously formed when oxygen is present. The understanding of the effect of incorporated oxygen on phase stability, elastic properties and bond distribution allows for quantum mechanically guided design of materials with improved oxidation resistance and stability at elevated temperatures. In the first part, oxygen incorporation in the MAX phases Ti2AlC, V2AlC and Cr2AlC was studied by ab initio calculations. Comparing calculated energies of formation for oxygen incorporation indicates that oxygen substitutes carbon in Ti2AlC and V2AlC, but is incorporated interstitially in the Al layer of Cr2AlC even for carbon deficient Cr2AlC. To evaluate these predictions, combinatorial DC sputtering was used to deposit thin films with different oxygen concentrations. Two phase-regions of Cr2AlC and Cr2Al were investigated in order to study oxygen incorporation in carbon deficient Cr2AlC. X-ray strain analysis data indicate that the a and c lattice parameters increase with oxygen content. These trends are in good agreement with the change of lattice parameters predicted by ab initio calculations and therefore corroborate the notion of interstitial oxygen incorporation in Cr2AlC. A metastable solubility limit for oxygen of 3.5 at.% was determined experimentally. This is the first report on interstitial oxygen in a MAX phase and may be of relevance during the initial stages of oxidation. In the second part, various point defect configurations were studied by ab initio calculations in terms of formation energies, equilibrium volume and elastic moduli to identify the origin of the experimentally observed nitrogen over- and understoichiometry and as a starting point for studying oxygen incorporation in TiAlN thin films. From formation energies and comparison to existing experimental equilibrium volume and elasticity data, it was shown that nitrogen vacancies and metal vacancies are responsible for nitrogen understoichiometry and overstoichiometry, respectively. Irrespective of the type of vacancies, the bulk modulus is decreased by approximately 7% as the nitrogen concentration is increased or decreased by 3 at.%. The third part deals with the impact of oxygen additions on phase stability and elastic properties of TiAlN. Based on ab initio calculations, it was shown that fcc-Ti0.5Al0.5N and fcc-Ti0.5Al0.5O show a negative energy of mixing. The lattice parameter of Ti0.5Al0.5N1 xOx increases with increasing x while the bulk modulus decreases by up to 35%. The reason is the metallic bonding character which increases at the expense of the covalent character with increasing oxygen content. It was shown that the incorporation of oxygen in Ti0.5Al0.5N1 xOx on interstitial sites is energetically favourable. This can be understood based on the compensation of metallic bonding as the population of metal-oxygen bonds that are similar to alpha-Al2O3 and rutile-TiO2 is increased. Based on the oxygen incorporation induced stability increase of Ti0.5Al0.5N1 xOx+y, a new design concept for transition metal oxynitride based hard coatings was developed. In order to test the theoretical predictions made regarding the non metal substitutions, a fast synthesis method was developed where reactive gas gradients are employed during combinatorial thin film deposition. This method is evaluated in the fourth part of this thesis. N2 and O2 partial pressure gradients over the target surface were used to deposit TiNx, TiOy, TiAlNx and TiAlNxOy thin films with varying x and y in a single deposition. It is shown that reactive gas gradients can be used to synthesize films with lateral composition gradients and that such films are useful to study their composition-structure-properties relationship. A new amorphous or nanocrystalline hard TiOy (y < 1) phase is reported as well as an increase of the (200) lattice spacing of TiAlNxOy that may indicate interstitial oxygen incorporation. Additionally, in the fifth part, an extension to the rule of mixture (ROM) to the sub unit-cell level is reported, deriving how bulk moduli of single phases with layered crystal structure and solid solutions can be reliably estimated. This method was tested for MAX phases and MnAl4C3+n phases. Based on ROM, DFT combinatorics was developed, reducing calculation time for elastic properties by some orders of magnitude. This is a significant step towards knowledge-based materials design since elastic properties can be predicted based on an efficient high throughput methodology.