Some Background

Basic info that you should know before going full steam ahead:
NYO-USA (which I will from now on just call NYO--you'll probably get a lot of NYO-Great Britain hits if you just search up NYO though) is a relatively new program that Carnegie's Weill Music Institute started the summer of 2013. They take about 120 musicians each year, including apprentices (which are like internships but with a music audition part), and tour all over the world, to a new country/countries each summer. This year, 2015, we are going to China, from Beijing to Hong Kong. Last year they did a coast-to-coast home tour of the US, and the inaugural year they went to Russia and England, ending at BBC Proms! The program is roughly a four-week course, with the first half residency training at Purchase College, and the second half of course touring.
NYO commissions a new piece every season to perform on tour, and the composer
along with a guest conductor and guest soloist (all new every year so far) come up to New York to rehearse with during residency. So we usually have one symphonic piece, a concerto of some sort for the soloist, and the commissioned piece, as well as at least one encore piece (IDK if I was supposed to tell you that?). I really loved last year's encore, which was a Porgy and Bess arrangement, probably arranged by the orchestra librarian and apprentice. Tbh there's a load of videos that NYO has uploaded that are just fabulous, like the Star-Spangled Banner, and a Happy Birthday arrangement for Maestro David Robertson's birthday last year, so I couldn't possibly begin to list my favorites. Just go check them out when you have time.
I'm sure I'm forgetting a whole lot of important things so I'll just make new posts for them as I go along, but I'd just like to end by talking about the instrumentation. There are more than enough musicians in each of the standard sections, but for the more auxiliary instruments, auditions and availability change from year to year. Saxophone, for example, has only ever been needed once so far, and that was last year, with Chad Lilley from Olney, Maryland representing. As for auditions themselves, you have to submit video recordings of your solo and excerpts, as well as a short response to a question, but you aren't allowed to audition if you are majoring in music or are enrolled in a conservatory. I think their purpose for that is to let people who won't make music their whole life have an opportunity like the ones those music majors will have. However, plenty of people are about to graduate into conservatories, like Juilliard and NEC, and that's perfectly legit. So you just can't be in one at the moment.
I hope this clears things up a bit; check out their website here for way more info. They also have a new thing this year where there's a directory of alumni so you can call/email them for their input, and I think that's really cool. I, for now, am a "#nyoobie," so I'm not on the list yet--but I will be. :) So as always, stay tuned, and comment any and all of your questions!