It's a set phrase that comes from the word 疲れる(tsukareru), which means "to exhaust". (疲れた(tsukareta)=be exhausted) And it's used when you want to thank someone for their hard work, for example after work or after school. Hope it helped ^^

I would like to answer your question. “おつかれさまでした” means good work. When you are leaving in a work place, you can say “おさきにしつれいします”. And other people will say to you “おつかれさまでした”. “おつかれさまでした” can be used in both formal and informal situation. Adults like to use this phrase. But I think children don’t use it. I hope I can help with your Japanese studies.

One more thing; おつかれさまでした。おさきにしつれいします literally meaning "I am leaving earlier than you"。Casually: おつかれ。おさきです。

Correctly, おつかれさまでした