In a rhombus, the diagonals bisect each other at right angles and also bisect the angles of the rhombus.
Given that \( \angle MNL = 55^\circ \), we recognize that:
- The diagonals LN and MO intersect at point P, creating \( \angle MNL \) and \( \angle MNP \) as parts of \( \angle MNO \).
- Because diagonals bisect the angles, \( \angle MNL = \angle MNP = 55^\circ \).
To find \( \angle MNO \), we use the fact that the two angles \( \angle MNL \) and \( \angle MNO \) form a linear pair and must sum to \( 180^\circ \). This gives us:
\[ \angle MNO = 180^\circ - \angle MNL - \angle MNP = 180^\circ - 55^\circ - 55^\circ = 180^\circ - 110^\circ = 70^\circ. \]
Therefore, the measure of angle \( \angle MNO \) is:
\[ m<MNO = 70^\circ. \]