I've done the OT in August once (nice and slow over 8 days was very relaxing). We shared a hut on the first two nights (Windy Ridge and Kia Ora) with a couple on their honeymoon (bad luck for them), but had the remaining huts to ourselves for the rest of the trip. Mind you, this was about 10 years ago. I would expect you'd see more people in winter these days, especially in the Pelion area.
We tool cross-country skis with us, but it was a winter of not a lot of snow so we ended up using them less than planned. They were great for where the snow cover was good though. However, I've since tried snow shoes, and they are a lot more practical this kind of walking (ie, where you cannot ski the whole way), since they are easier to carry, and easier to get on/off.
Walking in deep snow without snow shoes is very slow and exceptionally tiring, and I'd strongly recommend getting snow shoes if you can. It makes walking across the top of the snow just as easy as walking over grass.
Of course you need to be prepared for severe weather. We didn't get anything too extreme, but just the normal snow, sleet, wind, rain, hail and thunderstorms. Oh, and of course it was very cold!

We did get to ski Mt Ossa and Barn Bluff though. That was fun!
We carried a tent (for safety), but ended up sleeping in huts every night. I usually avoid the huts, but when they're not full of people, they're really quite nice. Do NOT count on the huts being warm. The new Waterfall Valley hut was the only one with a gas heater at the time but it was all out of gas when we went through, and we had two VERY cold nights there (after looking forward to a warm gas heater for 6 days). The coal heaters in the older huts can be difficult to get going, if you're not familiar with them. People say that if it is exceedingly cold, you can pitch your tent inside the hut for a warmer night's sleep (obviously only if you have the hut to yourself).
The scenery, of course, is fantastic, and well worth it when (if) you get breaks in the weather. Due to the time of year, you should expect fewer and shorter breaks in the weather, but when you do get them, it's just magic.
Beware of ice on the track too. Very slippery indeed!
