"ought to" and "should" are very similar in meaning. However, "ought to" is more formal. "Should" is much more common in American English. "Ought to" is sometimes used in England, but is still out used by "should." As for when to use them, they can be used interchangeably. There ought to be more people working at the till. There should be more people working at the till.

In American English, all questions, as well as negative statements, use “should” rather than “ought to.” (In UK English, this is not necessarily so, in which case, they would omit the “to” and treat “ought” as a “modal auxiliary.”) Examples: Should I? No, you shouldn’t. Affirmative statements can use either “ought to” or “should” — there is no difference in meaning.

There is little or no difference.

there is no difference in my opinion. Searching "ought to vs should" on Google may find a specific answer.