We present results of the photometric (from 3 to 509d post-explosion) and spectroscopic (up to 230d post-explosion) monitoring campaign of the He-rich Type IIb supernova (SN) 2015as. The (BV) 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 22d after the explosion, at an absolute magnitude of 16.820.18mag. At 75d after the explosion, its spectrum transitions from that of a SN II to a SN Ib. P Cygni features due to Hei lines appear at around 30d after explosion, indicating that the progenitor of SN 2015as was partially stripped. For SN 2015as, we estimate a 56Ni mass of 0.08M and ejecta mass of 1.12.2M, 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 (0.1M), a nearly compact (0.051013cm) H envelope on top of a dense, compact (21011cm) and a more massive (1.2M) He core. The analysis of the nebular phase spectra indicates that 0.44M of O is ejected in the explosion. The intensity ratio of the [Caii]/[Oi] nebular lines favours either a main-sequence progenitor mass of 15M or a WolfRayet star of 20M.