Hey BB,
I don't claim to be an expert but have been on a couple oryx hunts. Oryx are fairly tough critters to field judge but once you've looked at some mounts, pictures, and animals in the field you can get a feel for a big'un.
One thing you'll soon notice is for the most part the cows have thinner horns than the bulls. Oryx are one species where the mature cows can often have longer horns than bulls. Before your hunt you may want to consider whether you want to shoot a longer horned cow or a shorter, more massive horned bull.
On my hunt the best bull I could find had about 37" horns and I found 4 cows in the 39 to 41+" range. After thinking it over I couldn't get the 41"+ cow out of my head so went back and shot her. I am still happy I decided on the cow because there aren't a whole lot of oryx that get over 41" and to me I consider her a real trophy.
I have never liked using eyes, ears, nose measurements to judge game. More times than not a particular critter you shoot will have short ears or a stubby body part that screws up your estimation. As an example a lone yearling or 2 year old oryx may trick you because his body parts are smaller proportionately to his horns. However, if you are looking at a group of 20 oryx it is pretty easy to tell which ones are cows and which is the longest, most massive bull. A long horned, massive bull will blow your mind and you won't be second guessing whether to shoot or not!
The on-range NMex hunts are only a couple days so this doesn't leave you much time to dink around. Believe me, if the weather goes fowl the oryx can hide and can be extremely tough to find. Hunting pressure can also do this so you need to remember that although you see lots of oryx on opening morning they may be scarce to find after the first 1/2 day!
If you want to email me I can probably fill you in on a bunch of other things that may help you on your hunt. I'd also be happy to send you a couple pics of my oryx and how I had her mounted.
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