By means of the ultrafast time-resolved optical spectroscopy we investigated the effect of different organic molecular semiconductors on the magnetic anisotropy of cobalt thin films as a consequence of forming a ferromagnetic-metal/organic-semiconductor spinterface. In addition to the molecules, the effect of natural cobalt surface oxidation was also investigated for comparison. We did not find large differences in the behavior of the samples containing molecules of different geometries. The molecules were found to significantly increase the in-plane magnetic anisotropy of the films below the temperature of $\sim$170 K. Additionally, the magnetic anisotropy inhomogeneity increased. The in-plane magnetic anisotropy also increased in the naturally oxidized sample below the temperature of $\sim$100 K. The increase was much smaller then in samples with organic molecules.