Is world series common or proper noun?

The noun 'Olympics' is a proper noun, a shortened for the the proper noun 'Olympic Games', the name of a specific international sporting event.

A proper noun is always capitalized.

A common noun is a general Word for any person, place, or thing. Examples of common nouns for the proper noun 'Olympics' are competition, event, or occasion.

A common noun is capitalized only when it is the first Word in a sentence.