  We investigate the impact of combining Gaia astrometry from space with
precise, high cadence OGLE photometry from the ground. For the archival event
OGLE3-ULENS-PAR-02, which is likely a black hole, we simulate a realistic
astrometric time-series of Gaia measurements and combine it with the real
photometric data collected by the OGLE project. We predict that at the end of
the nominal 5 years of the Gaia mission, for the events brighter than
$G\approx15.5$ mag at the baseline, caused by objects heavier than 10
$M_{\odot}$, it will be possible to unambiguously derive masses of the lenses,
with accuracy between a few to 15 per cent. We find that fainter events
($G<17.5$) can still have their lens masses determined, provided that they are
heavier than 30 $M_{\odot}$. We estimate that the rate of astrometric
microlensing events caused by the stellar-origin black holes is $\approx 4
\times 10^{-7} \, \rm yr^{-1}$, which implies, that after 5 years of Gaia
operation and $\approx 5 \times 10^6$ bright sources in Gaia, it will be
possible to identify few such events in the Gaia final catalogues.
