One of the key parameters in determining the power conversion efficiency (PCE) of bulk heterojunction (BHJ) organic solar cells (OSCs) is the open circuit voltage (VOC). The processes of exciting the donor and acceptor materials individually in a BHJ OSC are investigated and are found to produce two different expressions for VOCs. Using the contributions of electron and hole quasi-Fermi levels and charge carrier concentrations, the two different VOC expressions are derived as functions of the energetics of the donor and acceptor materials and the photo-generated charge carrier concentrations, and calculated for a set of donor-acceptor blends. The simultaneous excitation of both the donor and acceptor materials is also considered and the corresponding VOC, which is different from the above two, is derived. The VOC calculated from the photoexcitation of the donor is found to be somewhat comparable with that obtained from the photoexcitation of the acceptor in most combinations of the donor and acceptor materials considered here. It is also found that the VOC calculated from the simultaneous excitations of donor and acceptor in BHJ OSCs is also comparable with the other two VOCs. All three VOCs thus derived produce similar results and agree reasonably well with the measured values. All three VOCs depend linearly on the concentration of the photoexcited charge carriers and hence incident light intensity, which agrees with experimental results. The outcomes of this study are expected to help in finding materials that may produce higher VOC and hence enhanced PCE in BHJ OSCs.