In this talk I will discuss the effects of single derivative mixing in massive bosonic fields. In the regime of large mixing this leads to striking changes of the field dynamics, delaying the onset of classical oscillations and decreasing the effects of friction due to Hubble expansion. I will discuss how this can change the cosmological evolution of the axion field in the presence of primordial magnetic fields, and show that this increases the axion abundance allowing heavier axions to make up all of dark matter.