OAHM: A Fortran subroutine for solving a class of unsymmetric linear systems of equations. The discretization, by means of the usual 5-point difference formulae, of an elliptic partial differential equation can give rise to an unsymmetric and sparse linear system of equations in which the matrix A of coefficients is an M-matrix such that each element in the upper/lower triangular part is greater than or equal to the respective element in the lower/upper triangular part. In other applications, dense matrices with the same property can arise. A FORTRAN subroutine called OAHM is presented for the solution of such systems which can be easily modified to deal efficiently which the sparse case. The OAHM subroutine uses a regular splitting and the convergence is therefore guaranteed.