<mo> on your character model will only work if you are self-clickable. The setting is in /characterconfig, Control Settings, Mouse, Enable clicking on self. I personally don't like being able to click on self, so I leave it turned off and just mouseover the party frame. <mo> will always work on the party frame.

Because of how heals redirect when the target is invalid, <mo> will target yourself if your mouse is over an NPC or enemy. When soloing, it helps to put your mouse over your target frame or the party frame and just leave it there. Doing <mo>,<t> will do what you want it to do when soloing, but will exhibit some unwanted side effects in party play if you are acting just a bit too quickly, as when the <mo> line tries to execute just before the GCD finishes, it'll skip to the <t> line and you'll heal yourself. It's best to use <mo> only.