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.
Sometimes we forget to add things. We are human, you know...not to mention volunteers who put a lot of time into this...
I appreciate you helping us throughout this course Alex. Keep up the good work!
It is just wrong. The verb "to happen" cannot take a direct object.