In this work, I have investigated the transient electronic structure of Ni and NiO following an optical excitation using time-resolved X-ray absorption spectroscopy (tr-XAS) to reveal the role of electronic correlation during the non-equilibrium dynamics. These experiments have been performed using the novel beam-splitting off-axis zone plate setup at the "Spectroscopy and Coherent Scattering" instrument of the European XFEL. Through the high data quality achieved with this setup, I could apply a modelling-based analysis to identify and isolate the underlying spectral modifications of the pumped spectra, which allowed for a detailed comparison with theoretical calculations. Specifically for the measurements on Ni-metal, 800 nm optical laser pulses were used to excite 3d electrons into 4sp states, with the transient changes in the 3d states being probed after a set time delay by X-ray absorption measurements of the L2,3-edge. I combined this analysis with ab initio TDDFT calculations, revealing an interplay of local electron correlations with the spin-dependent non-equilibrium dynamics, which causes transient changes in the electronic structure. These results established tr-XAS as a tool capable of tracking the transient changes in the electronic structure and thus reveal the effects of electronic correlations during non-equilibrium dynamics. For NiO, I used 266 nm optical laser pulses to resonantly excite electrons from the ligand 2p band into the upper Hubbard band. In contrast to 3d transition metals, the resulting photo-doped holes in the ligand band are not effectively screened. Consequently, the non-equilibrium dynamics are affected by both local correlations between d electrons and non-local correlations with photo-doped ligand holes. To examine this in detail, I investigated the changes in the Ni 3d and O 2p electronic density of states at specific time delays through X-ray absorption measurements at the Ni L2,3 and O K edges, respectively. Through a comparison with GW+DMFT calculations, I identified an interplay of local and non-local electronic correlations affecting the non-equilibrium dynamics. In addition, I identified a transient L3 pre-edge feature, which, via comparison with non-thermal multiple ligand field calculations, could be linked to d-d excitations. These results further demonstrated the capabilities of tr-XAS to not only resolve the influence of local correlation but also investigate the effect of non-local correlations on the transient electronic structure, which could be distinguished from additional multiplet excitations. The occurrence of long-lived excitations is one of the biggest challenges with high repetition rate time-resolved XAS measurements. These excitations cause additional modifications of the pumped spectra that build up with an increasing number of laser pulses, thus limiting the repetition rate or number of pulses in the experiment. In this thesis, I analyze and separate the contribution of these excitations and discuss methods by which their effect can be minimized or corrected.