This article is from the CD-Recordable FAQ, by Andy McFadden (email@example.com) with numerous contributions by others.
OPC stands for Optimum Power Control (or Optimum Power Calibration,
depending on who you believe; the process is sometimes known as Dynamic
Power Control (DPC) or Direct Read During Write (DRDW)). Most CD-R units
do a power calibration test before writing to adjust the laser power to the
correct strength. Different brands of media and different recording speeds
require slightly different power levels. Too much power can create oversized
marks which can interfere with each other, and too little power can produce
undersized marks which, in extreme instances, can cause read failures.
The recorder reads a recommendation for the initial power level from the
Recommended Optimum Recording Power value from the ATIP (section (2-38))
on the disc. This is used as a starting point for a series of write tests
in the Power Calibration Area (PCA) of the disc.
Running OPC goes a step farther by actively monitoring the write process
and adjusting the laser power as needed. If the writer encounters dust or
fingerprints, the laser power can be increased to burn through the
obstacles. This is especially useful for discs that are moved around
between recording sessions, such as CD-RW discs or multisession CD-Rs.
For more information, see the OSTA white paper on the subject
at http://www.osta.org/specs/pdf/opc.pdf. Another good site is