In this paper we investigate the one-dimensional harmonic oscillator with a leftright boundary condition at zero.This object can be considered as the classical selfadjoint harmonic oscillator with a singular perturbation concentrated at one point.The perturbation involves the delta-function and/or its derivative.We describe all possible selfadjoint realizations of this scheme in terms of the above mentioned boundary conditions.We show that for certain conditions on the perturbation (or, equivalently, on the boundary conditions) exactly one non-positive eigenvalue can arise and we derive an analytic expression for the corresponding eigenfunction.These eigenvalues run through the whole negative semi-line as the perturbation becomes stronger.For certain cases an explicit relation between suitable boundary conditions, the non-positive eigenvalue and the corresponding eigenfunction is given.