  The effects of uniaxial compressive stress on the normal state $^{17}$O
nuclear magnetic resonance properties of the unconventional superconductor
Sr$_{2}$RuO$_{4}$ are reported. The paramagnetic shifts of both planar and
apical oxygen sites show pronounced anomalies near the nominal
$\mathbf{a}$-axis strain $\varepsilon_{aa}\equiv\varepsilon_v$, that maximizes
the superconducting transition temperature, $T_{c}$. The spin susceptibility
weakly increases on lowering the temperature below $T$$\simeq$10 K, consistent
with an enhanced density of states associated with passing the Fermi energy
through a van Hove singularity. Although such a Lifshitz transition occurs in
the $\gamma$ band, formed by the Ru $d_{xy}$ states hybridized with in-plane O
$p_{\pi}$ orbitals, the large Hund's coupling renormalizes the uniform spin
susceptibilty, which, in turn, affects the hyperfine fields of all nuclei. We
estimate this \textquotedblleft Stoner\textquotedblright\ renormalization, $S,$
by combining the data with first-principles calculations and conclude that this
is an important part of the strain effect, with implications for
superconductivity.
