  Most massive stars are born in binaries close enough for mass transfer
episodes. These modify the appearance, structure, and future evolution of both
stars. We compute the evolution of a 100-day period binary consisting initially
of a 25 M star and a 17 M star, which experiences stable mass transfer. We
focus on the impact of mass accretion on the surface composition, internal
rotation, and structure of the accretor. To anchor our models, we show that our
accretor broadly reproduces the properties of {\zeta} Ophiuchi, which has long
been proposed to have accreted mass before being ejected as a runaway star when
the companion exploded. We compare our accretor to models of single rotating
stars and find that the later and stronger spin-up provided by mass accretion
produces significant differences. Specifically, the core of the accretor
retains higher spin at the end of the main sequence, and a convective layer
develops that changes its density profile. Moreover, the surface of the
accretor star is polluted by CNO-processed material donated by the companion.
Our models show effects of mass accretion in binaries that are not captured in
single rotating stellar models. This possibly impacts the further evolution
(either in a binary or as single stars), the final collapse, and the resulting
spin of the compact object.
