Older generations had to speak Japanese in school and work places when Korea was under Japanese colony (but it's all in the past ) and naturally, a lot of Japanese words are still used in Korea, even 50+ years after the colonization is over.
And here are some very common Japanese words still used in Korea.
How common? You can say these words and EVERY Korean person will understand what you mean.
다마네기(onion)
자부동(floor cushion)
우동(udon)
오뎅(oden)
사라(bowl)
요지(toothpick)
쓰메끼리(nail cutter)