That's odd, I've played that quest dozens of times (and again just now) and the Ogres are always visible.
If it's happening elsewhere, that suggests it's possibly due to missing/corrupt Ogre model/animation files. Since I can't replicate the problem you are having, I deleted my Ogre model/animation files and that did indeed make them invisible.
I'd recommend verifying game files
and seeing if that makes the Ogres visible.