Well the oil and water do not mix and if sitting long enough the water will separate from the oil and yes is can freeze, I believe the system is open enough and that the can is rigid enough to with stand the water from expanding outwards and deforming the can. Long story short, once ur engine heats up the ice should thaw and ur catch can will be fine. Of course the more full it is the more likely there would be cause for damage so make sure you don't let it get overfull in the winter months. Also the colder it gets the more condensation you are going to find in your catch can so definitely worth keeping an eye on but if you are emptying the catch can every oil change I am most certain you will be perfectly fine.