Is RS- more basic than RO-?
RobChem has already pointed out in his comment that your assumption is not quite correct.

Take a look at $\mathrm{p}K_\mathrm{a}$ values for $\ce{ROH}$ and $\ce{RSH}$ in water from the CRC Handbook of Chemistry and Physics and/or other online sources, such as this or that.

\begin{array}{lrr}
\mathbf{R} &amp; {\ce{\mathbf{OH}}} &amp;\ce{\mathbf{SH}}\\
\hline
\ce{H} &amp; 15.7 &amp;7.0\\
\ce{Et} &amp; 15.9 &amp; 10.6\\
\ce{(H3C)3C} &amp; 18.0 &amp; 11.7\\
\ce{C6H5} &amp; 9.9 &amp; 6.6\\
\end{array}

In all the cases above, the $\mathrm{p}K_\mathrm{a}$ value for $\ce{RSH}$ is smaller than that of $\ce{ROH}$.