  When the planet formation process begins in the disks surrounding young stars
is still an open question. Annular substructures such as rings and gaps in
disks are intertwined with planet formation, and thus their presence or absence
is commonly used to investigate the onset of this process. Current observations
show a limited number of disks surrounding protostars exhibiting annular
substructures, all of them in the Class I stage. The lack of observed features
in most of these sources may indicate a late emergence of substructures, but it
could also be an artifact of these disks being optically thick. To mitigate the
problem of optical depth, we investigate substructures within a very young
Class 0 disk characterized by a low inclination using observations at longer
wavelengths. We use 3 mm ALMA observations tracing dust emission at a
resolution of 7 au to search for evidence of annular substructures in the disk
around the deeply embedded Class 0 protostar Oph A SM1. The observations reveal
a nearly face-on disk (i$\sim$16$^{\circ}$) extending up to 40 au. The radial
intensity profile shows a clear deviation from a smooth profile near 30 au,
which we interpret as the presence of either a gap at 28 au or a ring at 34 au
with Gaussian widths of $\sigma=1.4^{+2.3}_{-1.2}$ au and
$\sigma=3.9^{+2.0}_{-1.9}$ au, respectively. The 3 mm emission at the location
of the possible gap or ring is determined to be optically thin, precluding the
possibility that this feature in the intensity profile is due to the emission
being optically thick. Annular substructures resembling those in the more
evolved Class I and II disks could indeed be present in the Class 0 stage,
earlier than previous observations suggested. Similar observations of embedded
disks in which the high optical depth problem can be mitigated are clearly
needed to better constrain the onset of substructures in the embedded stages.
