  Supernova (SN) 2021ocs was discovered in the galaxy NGC 7828 ($z = 0.01911$)
within the interacting system Arp 144, and subsequently classified as a normal
type-Ic SN around peak brightness. VLT/FORS2 observations in the nebular phase
at 148 d reveal that the spectrum is dominated by oxygen and magnesium emission
lines of different transitions and ionization states: O I, [O I], [O II], [O
III], Mg I, and Mg II. Such a spectrum has no counterpart in the literature,
though it bears a few features similar to those of some interacting type Ibn
and Icn SNe. Additionally, SN 2021ocs showed a blue color, $(g-r) \lesssim
-0.5$ mag, after the peak and up to late phases, atypical for a type-Ic SN.
Together with the nebular spectrum, this suggests that SN 2021ocs underwent
late-time interaction with an H/He-poor circumstellar medium (CSM), resulting
from the pre-SN progenitor mass loss during its final $\sim$1000 days. The
strong O and Mg lines and the absence of strong C and He lines suggest that the
progenitor star's O-Mg layer is exposed, which places SN 2021ocs as the most
extreme case of massive progenitor star's envelope stripping in interacting
SNe, followed by type-Icn (stripped C-O layer) and Ibn (stripped He-rich layer)
SNe. This is the first time such a case is reported in the literature. SN
2021ocs emphasizes the importance of late-time spectroscopy of SNe, even for
those classified as normal events, to reveal the inner ejecta and progenitor
star's CSM and mass loss.
