"Large non-reactive saucepan" - doesn't sound like a pressure cooker to me.

Oxtail was specifically cut into 2 inch joints, as it always is.

The liquid included beef stock, a lot of tomatoes (peeled and chopped), a fair amount of onion and some herbs. After removing the finished oxtails this was to be boiled down to about 1/3, lightly thickened and used for sauce.

While oxtails aren't particularly fatty, with 4 pounds of them there was at least a half cup of fat floating on top - no mention of this. Of course, I strained the liquid out and pressed the vegetables to extract as much liquid as possible. I then defatted the liquid using my gravy separator.

I placed both liquid and solids in a saucepan and boiled it down to a little less than a third before thickening with 2 t Arrowroot - worked fine, but without defatting, the sauce would have been at least 1/3 fat - not good.

I will have my recipe up in a day or so with these corrections. It worked out very well my way.