The software controlled OK indicator may be labeled ACT for activity on some boards.
We've simplified the instructions offered on the Pi discussion board explaining how to blink the OK LED from the command line. post 
See Node Access to GPIO for our more general inquiry.
The OK LED is exposed as a file buried deep in the /sys directory. First go there.
cd /sys/class/leds/led0
We'll need write access to two files. Grant it.
sudo chmod a+w trigger brightness
A kernel module currently blinks OK when accessing the SD card. Disable this functionality.
echo none >trigger
Write 1 or 0 to brightness to turn OK on or off.
echo 1 >brightness echo 0 >brightness
Now we will write a loop to blink OK endlessly.
while true do echo 1; sleep .1; echo 0; sleep .1 done >brightness
Restore the behavior of the mmc0 kernel module.
echo mmc0 >trigger
Notes
Read about Easy Macro Photography used to take the close-up circuit board picture.