Weather - During summer, temperatures can reach 110F (42C), but on average, it's probably in the 90sF (30+C). It's hot and dry. During winter, temperatures are often around 20 - 25F (about -8C). Many buildings have poor or no heating systems. Warm layers are your best bet in winter (undershirt, turtleneck, sweater, fleece). A good pair of comfortable, warm boots is important. A good pair of gloves is also nice to have since most gloves sold here are just knit gloves, which aren't as warm as others sold back home (thinsulate, leather, etc.). It snows in Osh, but not a lot (from a Michigan, USA point of view). The difference is that in Osh, the sidewalks and roads are not salted so they can become sheets of ice and the snow can pile up when not swept away. Many women here wear high-heeled boots in the winter, which seems like they're just asking for a fall - especially at the bazaar. Boots with good traction would be my suggetion, but if you're a woman who places fashion above comfort, you can go with a high-heeled pair. Men usually wear rubber boots that look like fishing waders and they often tuck their pants inside them. I'd go with a pair from home because the boots here don't really have any insulation. You can find decent boots here (more likely in Bishkek), but it takes a long time and you may find that a good pair costs as much here as at home.
Women - Older women wear moo-moo type dresses, robes, slippers, and head scarves. Younger women/girls (mostly 30 and under - before they're married) tend to wear tight jeans and tops. You can bring belly-bearing, hip-hugging jeans, and wear them in Osh without any problems. Not many women/girls wear shorts, but you do see some around Osh, along with girls wearing really short skirts. A good bet for summers is crop pants, just-past-the-knee cargo pants, or skirts since they are cooler than jeans, but you don't have to worry about feeling uncomfortable (like if you're the only one wearing shorts). As a general rule, I would say women over 30 wear skirts past the knee (not necessarily ankle length) and women under 30 wear them above the knee. In places like Almaty and Bishkek, the women/girls often dress in very skimpy clothes. See-through clothing with exposed bras is not uncommon in those cities. Osh is more conservative. You don't see many see-through clothes or fish-net stockings. While there are women fully covered, you also see women/girls in western clothing. You can dress in normal "western" clothes or tight clothes and not have to worry about offending anyone. In the center of Osh, you can wear almost anything because they are used to seeing foreigners and people from Bishkek there. However, if you go ito a Mahalla (an Uzbek neighborhood), it is best to dress more conservatively.
Men - Men tend to wear jeans with printed t-shirts (not white undershirt t-shirts) or khakis/dress pants and button up, collared shirts or polos. Many older men wear blazers/suit coats, but more for warmth than formality. Young men (under 30) sometimes wear shorts in the summer, but in general guys over 30 wearing shorts are foreigners. Many men wear really pointy shoes here as their everyday shoe rather than sneakers or oxfords (they click like women's shoes at home do when they walk). Unless you're willing to embrace the pointy, clicky shoe as the locals have, I would suggest bringing comfortable shoes from home.
Cost of Clothes - You can find almost anything if you look hard enough - especially if you go to Bishkek (about US$100 round trip by air, much cheaper in a shared car, but the ride is about 14 hours). However, a decent pair of jeans can cost US$25 - $30 and you'll probably find they are poorer quality than what you would find at home if you spent the same. You can buy a shirt or sweater for about US$12, but it may shrink or fall apart relatively quickly. The cost may not seem high, but when you think of it in terms of the average local salary being about US$20 - $50 per month, it's expensive.
Bring Your Favorite Clothes - Osh has one nightclub and lots of cafes where it is common to go out dancing. It's nice to have clothes you feel good wearing when you go out. Don't just bring old clothes you can leave behind. Bring some favorites that you feel good wearing out and feel comfortable wearing. You'll be thankful, especially when you're having a rough day and you miss home and you want to feel good.
Dry Cleaning - I don't know of any dry cleaner in Osh so it's best to bring clothes that you can wash. If you bring a suit that needs to be dry-cleaned, there are local people who can clean them for you (about US$3/suit). I'm not sure how they do it, but they come back in one piece. Also, clothes dryers are basically non-existant in Osh so Dryel really isn't an option either.
Note on Undergarments: Clothes are usually dried on outdoor clothing lines so if you're a fan of unusual undergarments, keep in mind your neighbors will probably know about it.