I always considered onesies like underwear, the bottom layer of clothes, basically protecting baby's clothes from the diaper. So I would put a onesie on under tights and then put a dress on top.
The other day I saw a family w a young toddler girl (like 16 mos) wearing a shirt that was short enough that I could see she also had knit tights and a onesie on top of them. It looked so weird, like the onesie was supposed to be the pants.
The mom and baby also were wearing what appeared to be brand new Ugg boots, (which is strange in Wisconsin in March- we are thinking it is spring now) so perhaps they were confused by cold weather clothing choices.