The office staff claim or claims that they have been treated badly by management.
Which one should I use singular or plural?

Question

Answer

In American English, usually singular (although it would be interpreted as referring to more than one person). In British English probably plural. If we want to eliminate the issue, we can say "staff members".

Answer

Claim is coorect.

Answer

It depends. Very often in British English collective nouns are plural, while in American English they are usually singular.

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