adverb
BAD : Remember to get there early otherwise you may not get a seat.
GOOD : Remember to get there early. Otherwise you may not get a seat.
GOOD : Remember to get there early or you may not get a seat.
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Unlike or, otherwise is an adverb (NOT a conjunction): 'I'm glad that you told me about the show being cancelled. Otherwise I'd have travelled all the way to Glasgow for nothing.'