  Oxygen vacancies (OV) are pervasive in metal oxides and play a pivotal role
in the switching behaviour of oxide-based memristive devices. In this work we
address, through a combination of experiments and theoretical simulations, OV
dynamics in $Pt/TaO_{2-h}/Ta_{2}O_{5-x}/TaO_${2-y}$/Pt$ devices. In particular,
we focus on the RESET transition (from low to high resistance), induced by the
application of electrical pulse(s), by choosing different initial OV profiles
and studying their kinetics during the mentioned process. We demonstrate that
by selecting specific OV profiles it is possible to tune the characteristic
time-scale of the RESET. Finally, we show that the implementation of gradual
RESETs, induced by applying many (small) successive pulses, allows estimating
the activation energies involved in the OV electromigration process. Our
results help paving the way for OV engineering aiming at optimizing key
memristive figures such as switching speed or power consumption, which are
highly relevant for neuromorphic or in-memory computing implementations.
