Is it ok for a junior to call your boss by the first name?

I usually call people by their title and last name unless they invite me to do otherwise, especially anyone senior to me in age or status.

I never shorten people's names or use a nickname unless they say it is ok.

So I'd either wait for your Boss to say how he/she wants you to address him/her or ask him/her.

only if they ask you to. Otherwise it is better to call them by their last name.