  The influence of the hydrostatic pressure P up to 0.95 GPa on the excess
conductivity and the pseudogap {\Delta}*(T) in optimally doped YBCO single
crystals (T_c = 91.1K at ambient pressure) is investigated by electrical
resistivity measurements. A pronounced enhancement of the pseudogap under
pressure of dln{\Delta}*/dP= 0.32, which is only a factor of 1.12 smaller than
in slightly doped single crystals, is revealed for the first time. This implies
a somewhat more moderate increase of the coupling strength in optimally doped
cuprates with increasing pressure. Simultaneously, the ratio
2{\Delta}*(T_c)/k_BT_c=5 at P=0 GPa, which is typical for high-temperature
superconductors with strong coupling, increases by 16% with increasing
pressure. At the same time, the pressure effect on T_c is minor: dT_c/dP=+0.73
K/GPa, whereas dln{\rho}/dP=-17% 1/GPa is comparable with that in lightly doped
YBCO single crystals. This suggests that the mechanisms of the pressure effect
on {\rho}(T) and T_c are noticeably different. Independently of pressure, near
T_c, the excess conductivity is well described by the Aslamazov-Larkin (3D-AL)
and 2D Hikami-Larkin fluctuation theories, exhibiting a 3D-2D crossover with
increasing temperature. However, the temperature interval T_c<T<T_{01}, in
which excess conductivity obeys the classical fluctuation theories, is
exceptionally narrow (~1.16 K). Nevertheless, a peculiarity at the temperature
T_{01}, up to which the wave function phase stiffness in the superconductor is
maintained, is clearly observed in the dependence {\Delta}*(T). Below T_{01} a
fast growth of {\Delta}*(T) is revealed for the first time. It can be
associated with a sudden increase of the superfluid density, n_s, that is the
density of fluctuating Cooper pairs (short-range phase correlations) forming in
the sample when T approaches T_c.
