"Bir kaza oldu."

That would be "Bir kazaydı." "olmak" basically means "to happen" or "to become." Sometimes, in different tenses and contexts, it can also have the meaning of "there is/was/has been/etc." It isn't the verb to be, but it sometimes has some similarities.

No, it means "O bir kaza geçirdi."

Sometimes we forget to add things. We are human, you know...not to mention volunteers who put a lot of time into this...

It is just wrong. The verb "to happen" cannot take a direct object.