The easy sign of rough weather ahead is when they start putting barf bags in the stairwells.
I don't generally take sea-sickness medication for Oasis ships, but I do take it with me just in case. On cruises where there seems to be a bigger likelihood of rocking and rolling (transatlantics, smaller ships, notoriously rough areas, etc), I will proactively take something like Bonine (non-drowsy) as a once-a-day precaution.