Jill: I am a westside Extension agent (recently retired) but in my  experience, it is most likely voles (field mice). Rabbits are a possibility as they also make clean cuts, but they don't usually leave pieces, they eat them! And I don't know if they could get through your fencing. So you can test the field mouse/vole by putting out some sturdy mouse traps baited with peanut butter in the evening. You should know soon. 

It is also possible that chipmunks or some other animal more common to Central Oregon are the culprits. Crow usually pull seedlings out and don't cut them, in my experience. 

Let me know what you find out. Chip Bubl  OSU Extension 
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It recently occurred to men that it might be cutworms which aren't too common in home gardens but are a problem in some years in commercial cabbage family crops. Here is a link to information about them. Chip

https://pnwhandbooks.org/insect/vegetable/vegetable-pests/hosts-pests/broccoli-brussels-sprout-cabbage-cauliflower-cutworm  

It recently occurred to me that it might be cutworms which aren't too common in home gardens but are a problem in some years in commercial cabbage family crops. Here is a link to information about them. Chip

https://pnwhandbooks.org/insect/vegetable/vegetable-pests/hosts-pests/broccoli-brussels-sprout-cabbage-cauliflower-cutworm