In the movie it doesn't look at all like CGI, it looks like a costume. I just think its strange that they did most all creatures CGI then they did Otmin as a costumed character.
Hmmm I think they partly used CGI in the development of General Otmin, such as with the hairs, but they might have used an actor to wear the costume like Andy Serkis with Gollum from LOTR.
Unless I heard wrong, an actor indeed plays General Otmin, but they also used CGI in different scenes. The poor man could only hear the awful whirring and grinding of gears in his head as he wore the costume, but I think he made a great performance as the Witch's general. Kudos.
It wasn't Weta it was KNB EFX that built the costumes, prosthetics etc. Weta was responsible for was the weapons, armour and some of the creature designs. Weta also didn't do the CGI. That was done by Rythm & Hues and Sony Image Works. Please give other people credit where they are due.
PS: I'm not trying to some off as mean in case you got that idea when you read it. I'm really a nice guy
EDIT: Here's a link to some info on Gen. Otmin of his weapons and how he was created.