  We present results of the photometric (from 3 to 509 days past explosion) and
spectroscopic (up to 230 days past explosion) monitoring campaign of the
He-rich Type IIb supernova (SN) 2015as. The {\it (B-V)} colour evolution of SN
2015as closely resemble those of SN 2008ax, suggesting that SN 2015as belongs
to the SN IIb subgroup that does not show the early, short-duration photometric
peak. The light curve of SN 2015as reaches the $B$-band maximum about 22 days
after the explosion, at an absolute magnitude of -16.82 $\pm$ 0.18 mag. At
$\sim$ 75 days after the explosion, its spectrum transitions from that of a SN
II to a SN Ib. P~Cygni features due to He I lines appear at around 30 days
after explosion, indicating that the progenitor of SN 2015as was partially
stripped. For SN~2015as, we estimate a $^{56}$Ni mass of $\sim$ 0.08
M$_{\odot}$ and ejecta mass of 1.1--2.2 M$_{\odot}$, which are similar to the
values inferred for SN 2008ax. The quasi bolometric analytical light curve
modelling suggests that the progenitor of SN 2015as has a modest mass ($\sim$
0.1 M$_{\odot}$), a nearly-compact ($\sim$ 0.05$\times$10$^{13}$ cm) H envelope
on top of a dense, compact ($\sim$ 2$\times$10$^{11}$ cm) and a more massive
($\sim$ 1.2 M$_{\odot}$) He core. The analysis of the nebular phase spectra
indicates that $\sim$ 0.44 M$_{\odot}$ of O is ejected in the explosion. The
intensity ratio of the [Ca II]/[O I] nebular lines favours either a main
sequence progenitor mass of $\sim$ 15 M$_{\odot}$ or a Wolf Rayet star of 20
M$_{\odot}$.
