I think most people at your age will have no problem keeping the conversation in english - and my experoence from hanging out with people I study with si that whenever we have a foreign student with us everyone just keeps talking english, even to each other - and we kinda just forget about it. We often end up keeping the conversation in english even after the foreign student leaves (until someone asks "Why do we keep talking english?"). Also most courses at the bigger universities are given in both english and norwegian so if you plan to come here to study it should not be a problem at all.

My experoence when talking with some people is that getting a job without beeing fluent in norwegian might be hard - but it is possible. And much easier if you are going after jobs that require a masters degree. And if you get a master in computer science you should not have a problem getting a job here in Norway.