I too love Pho and often make it myself, no oxtail or bones though, just a good cut of beef from Sam's Club. I usually get a roast that's marked down for quick sale, hack it up into a few chunks, salt & pepper it before broiling it a bit.
I throw in cinnamon, clove, star anise, onion and ginger into the boil. I also supplement it which chicken stock. Yeah, chicken stock in beef Pho sounds odd but many restaurants do the same, so does my Laotian mother-in-law. Depending on the batch size, I also add a few beef bouillon to richen the flavor.
There are a ton of great recipes to start with, then like our beer, jump into it and add your own flair. I really dig a lot of cinnamon (stick), clove, and star anise in my batch. It really adds fragrance and a wonderful taste.http://www.vietworldkitchen.com/blog/2008/10/pho-beef-noodle-soup.html