Well, all you really need for getting an undead army is the spell Animate Dead. There are tings that help like Desecrate for more undead HD per cast or Remove Paralysis for fast zombies but those are easy to get.
Basically, I'd go either Life or Bones Oracle, depending on whether you want to focus more on healing or undead. Personally, I'd say go Life Oracle and get the needed spells.
The Bones Mystery doesn't have many revelations that are useful, the best probably being undead servitude but since Animate Dead grants you command over the created undead anyway it's really just used to control other undead.
im sorry but the BEST undead riser oralce (maybe the best class at all doing it) is the JUJU mystory one.
hell a juju oracle with agent of the grave can hold the most hd of unded under control and juju zmbies ARE PWOERFULL.
You Can take a juju or bones oracle and give it some healing spells. Or depending on your story pehaps you Can even take a spirit guide juju oracle and use the life spirit, bit that wouldent fly in my game.