Answer
The chemical formula for the conjugate acid of $OH^-$ is:
$OH_2$ or $H_2O$.
Work Step by Step
To find the conjugate acid of a base, you just need to add a proton $(H^+)$ to the compound.
1. Add one hydrogen.
$OH^- --\gt OH{_2}^-$
2. Add "+1" to the charge of the compound:
$-1 -- \gt 0$
$OH{_2}^- --\gt OH_2$
This compound is normally represented by "$H_2O$".
