Implemented in the ongoing research of the semiconductor materials group at the IFM at the university of Linköping on SiC, effects influencing the carrier lifetime as a result of thermal oxidation of 4H-SiC have been investigated and correlated with existing models. This work presents the influence of the oxide parameters such as layer thickness, oxidation temperature and time on the carrier lifetime of different as-grown samples. In addition, the influence of various initial conditions during the preheating process in two different oxidation furnaces on carrier lifetime are presented. One is an Al2O3 tube furnace and the other is an inductively heated quartz tube furnace. Of interest here is that doping of the oxide with Na from the Al2O3 furnace have been shown to have a strong influence on the oxide growth kinetics. Photoluminescence (PL) and time-dependent PL (TRPL) have been used to determine the minority carrier lifetimes and deep level transient spectroscopy (DLTS) was used to investigate the deep-lying non-radiative combination centres and determine their concentration. The results have been compared to a theoretical simulation based on the work of Klein et al. to aid with characterization. The results presented here indicate the presence of two new defects at 370K and 460K, later identified as ON1,2, with peculiar properties and a general lifetime behavior which cannot be explained by Z1,2 alone. Several assumptions regarding the properties of these defects have been made based on the experimental findings in an effort to explain their origin and suggest further investigation of said defects.