In a rhombus, the diagonals bisect each other at right angles. This means that if you have a diagonal \( LN \), it divides the rhombus into two congruent triangles: \( \triangle LMN \) and \( \triangle LNO \).
Given that \( \angle MNL = 55^\circ \), we can find \( \angle MNO \) as follows:
- Since \( LN \) is a diagonal, it bisects the angles at \( M \) and \( O \).
- Therefore, \( \angle MNL \) and \( \angle MNO \) are equal.
So, \( m\angle MNO = m\angle MNL = 55^\circ \).
Thus, the measure of angle \( MNO \) is:
\[ m\angle MNO = 55^\circ \]