I saw in the Navy Reserves they do have it, but there are no job openings at this time for JAG officers.

But, on the official JAG website it says that once you are a JAG officer, it is a full-time job, no real type of reserves.

So who is right?

I assume you meant this page?

http://www.jag.navy.mil/Careers/Careers3.htm

Did you read the FAQ?--

14. WHAT IS THE SERVICE OBLIGATION OF NAVY JAG CORPS?
All commissioning programs require active duty service. (Bold added) Individuals who enter the Navy JAG Corps via the Direct Appointment or Student Programs are required to maintain their commission for eight years from the date of their superseding appointment into the JAG Corps as a 2500 designated officer. This includes a four-year active duty obligation, which commences when the officer reports to their first duty station following graduation from Naval Justice School. Following the completion of the active duty obligation, officers may serve in an inactive status. At present there is no annual drill requirement, nor any requirement to take any other active part in the Reserve Program once they leave active duty.

And reserve status while attending school is also acceptable. But, no, you cannot serve in the JAG Reserves without first serving on active duty.

8 years seems like a long time (4 active, 4 inactive or all 8 active).

What is the usual service time for a person in the armed forces?

Thats a standard enlistment. Total time is equal to an eight year obligation no matter how you serve your time.

Is JAG really competitive? I heard you have to apply just for OCS Pensicola. I am wondering if the same thing happens for OIS Newport.