I'm a left-handed calligrapher and a while ago I switched from using a straight holder to a right-handed oblique.
There were mainly two great sources that made me decide to do this.
Firstly the video of John DeCollibus with basic tips for left-handed calligraphers, that is already being shared in the Tips for Lefties section.
But the second source hasn't been shared here yet!
It's a great blog post from @Younghae
Personally I love to write like this now! At first it felt really akward, but soon I already noticed the advantages: the tines make better contact to the paper (so my hairlines look way better now) and I can move around my arm way better. Also I can see much better what I'm writing!
After two weeks of practicing a little daily, it already felt easier!
An important thing that I want to share is that you probably tend to twist your head at first to see the letters like you normal do. Don't do this, as this will give you pain in your neck!
Study the letter forms by looking at them with the paper turned 90 degrees too. It gave me a whole new insight in how letters are formed!
Maybe I should read books like that too now
I hope this information will help other lefties!