Step 1:
Ba(OH)$_2$ is a strong base, so it dissociates completely in water:
Ba(OH)$_2$ $\rightarrow$ Ba$^{2+}$ + 2OH$^-$
The concentration of OH$^-$ is twice the concentration of Ba(OH)$_2$:
[OH$^-$] = 2 $\times$ 1 $\times$ $10^{-2}$ M = 2 $\times$ $10^{-2}$ M
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