Leadership only cares about character level, which is just total HD number (4 in this ogre's case) and not much else. CR calculation can be complicated but is ultimately irrelevant to the feat, and similarly if you were to want to eventually advance this ogre to keep up with the leadership level caps, there are rules for turning Monster entries into custom "monsters" with class levels up to that cap you would also need to follow; but this does not change the fact that your "level cap" is going to be those initial 4 HD plus the level of whatever class(es) you give the ogre as they raise the character level 1:1 with the HD gained per class level.
The bestiary has a section for monster cohorts, there is a serious "up charge" included. Iirc a ghoul counts as 5th level and an ettin as 15th. Just using cr/level equivalents is much simpler for sure.
Note those charge ups are for using creatures with meaningful abilities. An Ogre is objectively worse than a normal PC in every way because they are basically just aristocrats without any skill benefits.
Good point.