The interview/hire process was extremely easy. I applied online and got a call back within two days from the hiring manager. I was asked a couple of easy questions over the phone (why did you apply, why do you think you're a good fit, what is your availability, and any holiday dates I would be able to work) and then asked to come in for an interview the next day.
My first interview was a group one (though it was only me and one other girl as the others didn't show up) at Old Navy. The questions were pretty straightforward, but there were a few interesting activities that we did (for one "question", the manager that was interviewing us threw story cubes down with pictures on them and asked us to write a story that incorporated Old Navy using the two pictures on the cube). I know that the first interview is generally the hardest with Old Navy. I've walked in on numerous other interviews while I've worked there, as they're held in the break room in the back of the store, and every time they seem to be different and unique. Management is always coming up with new questions and activities to ensure that they hire unique and interesting people.
The second interview was very easy. I was called back for the next day (I got my response literally 3 hours after I had my first interview) and had my interview in the same place with two other interviewers. We were asked slightly more specific questions that pertained more to our skills and talents than our personalities. We were also asked to give any questions that we had about Old Navy. All three of the people in my second interview got hired, and in my experience, everyone who was called back for a second interview was hired. It seems (at least in my eyes) that the second interview is more management ensuring that they made a good choice rather than further investigation, as the first interview is so unique and interesting that they don't really need to do any further questioning.
If you're being hired as seasonal, know that, in most cases, there is only one session due to the high need for employees, so try to perform really well in that. Offers are generally given on the spot or within the next 24 hours.
The most frustrating part of the hiring process for me was after I was offered the job. I got my offer at home the night of my interview, but scheduling orientation was awful. It took 3 weeks and numerous calls from me in order to get the date of my orientation, and I lost several weeks of work in which I had no other source of income because I thought I would start working at Old Navy.
If you want to be hired, simply be yourself. Old Navy hires interesting people who are good workers, and if you don't practice for the interview and simply go in and show who you really are, you have a pretty good shot at getting the job.