Potential energy functions of the OH molecule are investigated from small to large inter-atomic distances R. The electronic problem is treated using an efficient Full Configuration Interaction (Full CI) approach that avoids orbital jumps found usually in multi-configuration self-consistent-field followed by multi-reference configuration interaction calculations of excited states. The calculations are performed for all the doublet, quartet, and sextet OH molecular states, up to the O(2p34s 3S) + H(1s 2S) asymptote, and for the lowest O− + H+ and O+ + H− ionic states. Inter-atomic distances, ranging from 0.5 Å to 20 Å, are spanned with a very small step in order to describe accurately the avoided crossings between the adiabatic potential energy functions. The accuracy of the potentials at small and large R values is analyzed. These Full CI calculations provide for the first time a global description of the 40 lowest molecular states of OH, well suited for dynamical calculations. The resulting potentials are used to obtain first estimates of cross sections and rate coefficients for different inelastic processes through the multichannel approach. This method, based on a Landau-Zener formalism taking into account the ionic-covalent avoided crossings at large distances, gives reliable results for the most intense transitions. It is shown that the largest rate coefficients correspond to mutual neutralization and ion-pair production processes.