Well, given that we can see the stars and that the Oort "cloud" is closer than the nearest star, then the answer to your main question is obviously that the "cloud" is not opaque.
I think it is called a cloud because it consists of many individual, small "particles" that don't interact with each other. Also, the term cloud avoids giving the impression that the Oort objects are arranged in any disk-like configuration - they should be much more spherically symmetric. The word cloud also avoids giving the impression that the objects fall between two relatively tight orbital radius limits. i.e. The Oort cloud is not like the Kuiper belt.