Hey, congrats on getting in. I will defer to anyone who knows something about women's clothing, but that seems fine. Especially if you're planning on getting something in addition before OCI, which is probably wise, you're definitely good. In terms of events that I did throughout 1L where dressing up was required:
1. Orientation dinner
2. 1L summer interviews during winter
3. Firm events at restaurants throughout January and February
4. Mock interviews in March
5. Oral argument for LPS in April
Others may have done more, but you basically have a sprinkling of events throughout the year where you need to look nice.
Re: day-to-day wear, I went to Penn for UG and I'm sure people here on average dress better than people at Michigan State or something because your average student at Penn comes from a relatively well-off family and there's money to spend. But yeah, it was mostly Uggs and Northfaces when I was in college. Law students are all over the map but on average they dress a little better than in college -- 26 isn't 19, and being stuck at school all day makes sweatpants and flip-flops a more unattractive option than when you could wake up for a 9:00 A.M. class and head back to bed for the rest of the day. In short, though: you can wear whatever you want.
Hope that helps.