The standard Ogre has no class levels
Do I just add two more HD for them being large creatures?
Yes, that's exactly it. Zombies are slow, but tough to kill. If the base creature has no character/npc class, you simply take their HD, convert to d12s and add whatever additional HD from the template based on their size. The standard Ogre from Bestiary 1 has no class levels.
But this doesn't mean that all ogres won't have a level in a class, they are intelligent enough to take class levels, and this is evidenced by all the Ogre variants who have class levels, such as the Ogre Brute.
You don't stack the armor bonus
but the ogre already has a AC of 17 and an AC of 20 seems excessive for a zombie.
Yes, but that's because you shouldn't add that natural AC on top of the base creature's AC, you gotta recalculate their AC, which will be reduced by their Dexterity penalty (-2, so they get a -1 to AC), and the template entry says the following:
It uses all the base creature’s statistics and special abilities except as noted here.
Since Armor Class is noted, you read that up and apply whatever says in there, which is:
Armor Class: Natural armor is based on the zombie’s size:
Well, since it's based on the zombie's size, the natural armor from the base creature can be scratched, it's irrelevant once the creature became a zombie.
This can be confirmed by looking at published zombified creatures, such as the Zombie Storm Giant (going from +12 to +4), or the Zombie Hill Giant (going from +9 to +3).
Example of creation
To make things simple, let's take a look at the Ogre's stats and then apply the zombie template on it. I won't go into detail about everything that has to be changed, but this should help you understand how the process goes.
The ability scores will be as follow (applying +2 Str, -2 Dex and null Con):
- Str 23, Dex 6, Con -, Int -, Wis 10, Cha 10
Now, let's take a look at the AC entry:
- AC 17, touch 8, flat-footed 17 (+4 armor, –1 Dex, +5 natural, –1 size)
This will be changed as follows: natural AC is now +3 instead of +5, and the Dex modifier is now -2 instead of -1. Which gives us:
- AC 14, touch 7, flat-footed 14 (+4 armor, –2 Dex, +3 natural, –1 size)
Also, notice that they also lose their Medium Armor Proficiency, so either take out that armor, or leave it there and apply the Armor Check Penalty to the creature's attacks, it's up to you. Personally, I would leave it there as it looks creepier.
Finally, another thing to note is that Ogres already have Toughness, so you don't add that hp to them again. Neither you calculate that based on their constitution score (now nonexistent), but on their charisma score. So their HP won't get that much of an increase, going from 30 (4d8+12) to 30 (6d8+6), despite the HD upgrade.