Organic semiconductors are composed of π -conjugated polymers and molecules. There is a large variety of devices in which the active layers are organic semiconductors; among them are organic light emitting diodes and organic photovoltaic cells. When devices based on organic semiconductors are exposed to external magnetic field their performance changes. We have investigated magnetic field effects in low and high fields (up to 8 Tesla with high resolution of ~10^-4 Tesla at low fields) in organic light emitting diodes made up of homo-polymer organic semiconductors. In these devices we have measured magneto-conductance, magneto-electroluminescence, magneto-photocurrent and magneto-photoluminescence. In these low mobility substances magnetic field effects are attributed to the spin degrees of freedom. We explain the experimental results by studying the effect of magnetic field on polaron pairs composed of oppositely charged species with spin 1/2. In the low field regime (~10mT) the magnetic field effects are caused by the hyperfine interaction between the spins of the polaron pairs and spins of the many protons that are present in all organic substances. In the intermediate field regime we consider a mechanism that was not discussed in detail previously and is caused by the anisotropy of the g-factor in these low symmetry organic molecules. In  high fields (higher than 2 Tesla) we consider partial thermal spin polarization as the mechanism responsible for the effects. We developed a simplified quantum mechanical approach taking into account the above interactions. With this model we could fit our experimental data and draw the following conclusions. The exchange interaction between polarons that compose the polaron pairs is approximately 6 neV while for charge transfer excitons, in which the distance between the two polarons is smaller, it is around 0.1 μeV. The orientation of the g-tensors within the polaron pair was found to depend on the organic semiconductor. The life time of a polaron pair in these organic light emitting diodes was determined to be of the order of ~μs.