Nooo, never ever waste gold on training oratory unless you absolutely are against being a diplomat.
It doesn't matter your main class, as long as you are a diplomat, you do not need to spend any gold on getting your Oratory skill above 80%. You just need either a region you like, or a ton of treaties, and some patience.
Oh yeah, did I mention patience? A few years ago, more than two players one separate occasions told me that BM is a game of patience. And possibly repetition.
You want to be the best? Deal with the boring early stages. It gets easier, faster, and better after the first few hurdles. Really.
I had a character start as inexperienced, 5% oratory. At this point, she has all priest options, all diplomatic options. With the best result in praising for 4 hours, she at one point raised 5 loyalty. This all occurred recently after only about two months.
