I've been on several October trips and the weather can be all over the board. Warm and pleasant; hot and humid; very hot and humid; blazingly hot and humid. And it's also hurricane season, so be prepared for some rain.
Note that the weather can swing wildly within the same week! In 2009 I packed mostly shorts, sandals, and sleeveless tops, based on a seven-day weather forecast of mid-90s and really high humidity (this was mid-October!). Well, on days Seven to Ten, the temperatures dropped more than 30 degrees, almost overnight. I had only packed one pair of yoga pants, one long-sleeved jacket, one pair of sneakers, and no warm socks. Because it was pretty breezy, I was pretty cold! I tried to layer the best that I could, but ended up having to buy some fairly expensive WDW apparel (sweatshirt, warm socks, etc.) and was so mad at myself for not bringing more warm clothes. It's really hard to find warm/tall socks at WDW! Finally found some at the Japan pavilion at Epcot (they were the kind that you wear with traditional flip-flop type Japanese shoes, socks with a split toe. They cost $10 and I never wore them again!).
So I recommend bringing at least some items of clothing for each weather type. It might save you from having to buy clothes on your trip!