I've looked it up in an Grammar text book here is what it says:
Should and ought to both express advisability or suggestion and the two are interchangeable
in meaning. Some grammarians consider only should a pure modal and
argue that ought to be classified as a related structure. Regardless of how they are
classified, should and ought to are closely related semantically and generally taught
together in ESL/EFL texts. Ought to occurs less frequently in American English
than should, although there are regional variations.
Questions and negatives with should and ought to follow the first auxiliary rule.
However, in American English, not is rarely used with ought to, nor is ought to
generally used in questions.
In spoken speech, ought to is generally reduced and sounds like outta or
oughta, and may be written as such in dialogues to reflect spoken speech.
In its reduced form ought to (outta/oughta) can be considered more informal
"Tiller clutched Sairy’s arm as they followed Dallas. Florida trailed
them, kicking at rocks and tree trunks as she went. “Oughta just run
away right now,” Florida mumbled. “Oughta just bury us alive."
What do you think about it?
You should have left earlier.
You ought to have left earlier."