M.O. also known as Mommy Only.
Seems like most kids go through it a few times at different ages. M.O. is when the only person in the world who can do whatever is required, who can possibly soothe or feed or read to is, of course, Mommy. M.O. means lots of crying, clinging, separation anxiety and "no Daddy." It can truly be exhausting.
But secretly, deep down (really, way down) most Moms will admit that M.O. is also the ultimate confirmation that Mom really does know best.