Freehorses is the expert on fat mage, still doesnt have dex. You dont need it on mounted mage anyway mages have huge radius that they can aim. The horse has max turning with 10 dex if you are in 1st gait so all you have to do is slow down to turn and you dont turn much while running anyway playing mounted mage.Oghrim / Huergar - (Age 63 - Midget - Fat 150%)
Stats (some flexibility here - i did tests for 118 INT / 123 INT) 118/123 INT - 65-72 DEX - 21-23 STR - 98-101 CON - 40ish PSY
Above build provides decent speed boost on foot as well better handling on horse for those who still wants to play Oghrim and not being boxing training bag when unmounted.
Due to changes in INT curve it is not that much worth to go full obese, also from the perspective of mounted travel and even more important lack of turning the horse smoothly ability when dex is only 10 points I would advise to avoid having such low Dexterity (thats technically huge nerf to Obese Fat mage people tend not to notice in last patches).
INT of 123 or even 118 is good enough for being efficient healer as well as occasionally (depending on situation) to toss still very decent a dmg spells. Only light weapons like flakestone/jadeite daggers are preferred solution here, but we are winning better horse riding that is a must when it comes to Mounted Magery anyways.
Min-maxing is sometimes not the best option at least from my observation, just an option and opinion from me to consider by whom it may concern.
If you are going to play a 118 int mounted mage might as well play a 116/115int buldging sheeva which has enough movement speed to outrun most fighters still. A 400 max movement speed mage on foot is just dead anyways so just put everything into int to get more heals that would keep your mount alive and allow you to kill other horses in 2.5 spells.
77 dex on a ogh might as well be 10, If you are not faster then most players as a mage then the speed is just conveince that could be better used to do burst damage which is what mounted fat mage is all about. You have to get as much damage in as possible with each speed on mounted fat mage because of the double cast time.