I am of the opinion that the giraffe is the best leader, for it has a long neck and long legs, making it the tallest animal in the forest.
I guess the underlined part mean "I have this opinion ..." or "I think that ..." but how do you interpret "of" in the context? It's rather tricky for us because we never say it that way.
I know it's a fixed phrase but how do you interpret "of" in the phrase?
It's also more forceful or intensive.
My opinion is
I am of the opinion
My conviction is
I am of the conviction
Your entire being believes so.