  We investigate the production of gravitational waves during preheating after
inflation in the common case of field potentials that are asymmetric around the
minimum. In particular, we study the impact of oscillons, comparatively long
lived and spatially localized regions where a scalar field (e.g. the inflaton)
oscillates with large amplitude. Contrary to a previous study, which considered
a symmetric potential, we find that oscillons in asymmetric potentials
associated with a phase transition can generate a pronounced peak in the
spectrum of gravitational waves, that largely exceeds the linear preheating
spectrum. We discuss the possible implications of this enhanced amplitude of
gravitational waves. For instance, for low scale inflation models, the
contribution from the oscillons can strongly enhance the observation prospects
at current and future gravitational wave detectors.
