I had just landed at Oakland with my CFI on a marginal VFR
day in a Piper Warrior.  As a 70 hour pilot, I wanted some
practice at a class C airport.  We had landed on 15 and
after refuel, taxied west on taxiway "c".  Ground told us
to follow the traffic ahead of us, a jet which turned left 
at taxiway "j".  The jet stopped before crossing runway
9L, then proceeded across.  Ground told us to "veer right
and proceed without delay across the end of the runway."
Since we were at the intersection of "j" and "c", we
started across runway 9L.  Halfway across, ground yelled at
us as we noticed a large jet on 1/2 mile final for 9L.  We
hurried across and ground explained that we were supposed
to cross the end of runway 15 (continue along taxiway "c")
instead of crossing 9L.  
  There are three taxiway intersections in a triangle within
100 feet of both runways.  These are c-j, c-p, and j-p.  
The taxiway diagram shows "p" as extending from the end of
15, so we did not think this was "crossing the end of
the runway (15)."  
  The ground controller's not using runway numbers, not 
using taxiway letters, and casual reference to
crossing the end of the runway contributed to the 
incursion.  Also a factor was the limited visibility,
our unfamiliarity with the airport layout, our failure
to stop and look before crossing the runway (without
delay), and our fixation on "follow that jet."  As PIC
it was my final responsibility to ensure crossing the
runway was safe.  I should have used an airport diagram
during the taxi.