Consider a function $f$ defined by the relation
$$
f\left(\Omega_{A} \Omega_{B}\right)=f\left(\Omega_{A}\right)+f\left(\Omega_{B}\right)
$$
First, differentiate partially with respect to $\Omega_{B}$, and then with respect to $\Omega_{A}$. Integrate twice to show
$$
f(\Omega)=\text { const. } \ln \Omega+\text { const. }
$$