"I oughtn't GO", for example, without "to" is perfectly correct and accepted standard English in the States, Dominic:-)
That's exactly what I said. It's perfectly correct and standard on both sides of the Atlantic, as is the form with "to".
Foreigners aka non-native English speakers will often use/say "I oughtn't to go....", but it's not natural usage, as an English teacher, you can trust me on this one.
It's perfectly standard, too, and not a non-native mistake. Again, on both sides of the Atlantic.