  In this letter a late-phase spectrum of SN 2010lp, a subluminous Type Ia
supernova (SN Ia), is presented and analysed. As in 1991bg-like SNe Ia at
comparable epochs, the spectrum is characterised by relatively broad [FeII] and
[CaII] emission lines. However, instead of narrow [FeIII] and [CoIII] lines
that dominate the emission from the innermost regions of 1991bg-like SNe, SN
2010lp shows [OI] 6300,6364 emission, usually associated with core-collapse SNe
and never observed in a subluminous thermonuclear explosion before. The [OI]
feature has a complex profile with two strong, narrow emission peaks. This
suggests oxygen to be distributed in a non-spherical region close to the centre
of the ejecta, severely challenging most thermonuclear explosion models
discussed in the literature. We conclude that given these constraints violent
mergers are presently the most promising scenario to explain SN 2010lp.
