construction of 'went ranting on'

Question

"You knew?" said Harry. "You knew I'm a –– a wizard?"

"Knew!" shrieked Aunt Petunia suddenly. "Knew! Of course we knew! How could you not be, my dratted sister being what she was? Oh, she got a letter just like that and disappeared off to that –– that school –– and came home every vacation with her pockets full of frog spawn, turning teacups into rats. I was the only one who saw her for what she was –– a freak! But for my mother and father, oh no, it was Lily this and Lily that, they were proud of having a witch in the family!"

She stopped to draw a deep breath and then went ranting on. It seemed she had been wanting to say all this for years.

-- Harry Potter and the Sorcerer's Stone by J.K. Rowling

Is the phrase this construction of OALD: go on doing something –– to continue an activity without stoppingHe said nothing but just went on working?

Answer 1

Your understanding is almost but not quite correct.

If JKR meant merely that Aunt Petunia continued to rant, she would have said she “went on ranting”. The inverted construction “went ranting on” implies a new action—in the theatre we would say a new “beat”—in which Aunt Petunia, after her deep breath, sets off in a different “direction”.

This construction is similar to that in an earlier question you raised:

Harry [...] | sent it | zigzagging | away into the air.  
She         | went    | ranting    | on

Petunia goes herself rather than setting something else going, but otherwise the syntax is the same.