  We present an observational and numerical study of the borderline hyperbolic
comet C/2021 O3 (PANSTARRS) performed during its recent passage through the
inner Solar system. Our observations were carried out at OASI and SOAR between
2021 October and 2022 January, and reveal a low level of activity relative to
which was measured for other long-period comets. In addition, we observed a
decrease in brightness as the comet got closer to the Sun. Our photometric
data, obtained as C/2021 O3 approached perihelion on 2022 April 21, show that
the comet was much less active than what is usually expected in the cases of
long-period comets, with $Af\rho$ values more in line with those of
short-period comets (specifically, the Jupiter Family Comets). On the other
hand, the observed increase in the value of the spectral slope as the amount of
dust in the coma decreased could indicate that the smaller dust particles were
being dispersed from the coma by radiation pressure faster than they were
injected by possible sublimation jets. The analysis of its orbital evolution
suggests that C/2021 O3 could be a dynamically old comet, or perhaps a new one
masquerading as a dynamically old comet, with a likely origin in the Solar
system.
