5. Let $AB$ be a chord of $\odot O$, and $CD$ be a diameter of $\odot O$ that intersects chord $AB$. Let $M=$
$|S_{\triangle CAB} - S_{\triangle DAB}|$, $N=$ $2 S_{\triangle OAB}$, then ( ).
(A) $M>N$
D-
(B) $M=N$
(C) $M<N$
(D) The relationship between $M$ and $N$ is uncertain
5. (B).

As shown in the figure, construct $DE \perp AB, CF \perp AE$ (the feet of the perpendiculars are $E, F$), and let the extension of $CF$ intersect the circle at $G$. Connect $DG$. Since $\angle DGC=90^{\circ}$ (as $CD$ is the diameter), it follows that
$EFCD$ is a rectangle, hence $DE = GF$. Without loss of generality, assume $CF > FG$. Construct $OH \perp CG, OL \perp AB$. Then $H$ is the midpoint of $CG$, and
\[
\begin{array}{l}
CF - FG = CH + HF - FG = 2HF = 2OL. \\
M = \left|S_{\triangle CAB} - S_{\triangle DAB}\right| \\
= \frac{1}{2} AB \cdot CF - \frac{1}{2} AB \cdot DE \\
= \frac{1}{2} AB(CF - FG) = AB \cdot OL = 2S_{\triangle OAB}.
\end{array}
\]

Therefore, the correct choice is (B).