1. What does it mean?
올클 shortens 올클리어, all clear. During course registration it means successfully enrolling in all the classes you intended to get. In gaming and other contexts it can mean completing every stage or item in a set.
Also written or said as olkeul
2. The nuance
The relevant checklist comes from context. Campus 올클 usually means registration success, not that the student has already passed or finished those courses.
3. Examples
“수강 신청 올클해서 원하던 시간표가 나왔어.”
I got every course I wanted and ended up with my planned timetable.
“이번에는 필수 과목 하나를 놓쳐서 올클은 못 했어.”
This time I missed one required course, so I didn't get the full set.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.