  Comets in the Oort cloud evolve under the influence of internal and external
perturbations, such as giant planets, stellar passages, and the galactic tidal
field. We aim to study the dynamical evolution of the comets in the Oort cloud,
accounting for external perturbations (passing stars and the galactic tide). We
first construct an analytical model of stellar encounters. We find that
individual perturbations do not modify the dynamics of the comets in the cloud
unless very close (< 0.5pc) encounters occur. Using proper motions, parallaxes,
and radial velocities from Gaia DR2, we construct an astrometric catalogue of
14,659 stars that are within 50pc from the Sun. For all these stars we
calculate the time and the closest distance to the Sun. We find that the
cumulative effect of relatively distant ($\leq1$ pc) passing stars can perturb
the comets in the Oort cloud. Finally, we study the dynamical evolution of the
comets in the Oort cloud under the influence of multiple stellar encounters
within 2.5pc from the Sun and the galactic tidal field over $\pm10$Myr. We
considered two models for the Oort cloud, compact (a $\leq$0.25 pc) and
extended (a$ \leq0.5$ pc). We find that the cumulative effect of stellar
encounters is the major perturber of the Oort cloud for a compact configuration
while for the extended, the galactic tide is the major perturber. In both
cases, the effect of passing stars and the galactic tide raises the semi-major
axis of $\sim1.1$\% of the comets at the edge of the cloud up to interstellar
regions ($a >0.5$pc). This leads to the creation of transitional interstellar
comets, which might become interstellar objects due to external perturbations.
This raises the question about the existence of a cloud of objects in the
interstellar space which might overlap with our Oort cloud if we consider that
other planetary systems face similar processes for the ejection of comets.
