～うございます - keigo い-adjectives
I still remember the introductory lecture of the first Japanese course I took in college, my sensei told the class that おはようございます does not mean "good morning" but it is actually derived from はやいです which simply means "it is early".
Some time later, I've been told that this honorific formation of i-adjectives is not restricted to はやい only, but can be applied to any i-adjectives. I've not been told how though.
I'm assuming that for any i-adjectives that end with X-ai, they become o-X-ou:
はやい → おはよう
たかい → おたこう
But what about adjectives that end with -ui, -ii etc? Turning them into -ou does not sound right. For example:
さむい → おさもう?
たのしい → おたのしょう?
おおきい → おおおこう? [triple お]
What are the rules for turning i-adjectives into honorific form? And also, are this honorific still in use today and in what kind of scenarios?