This is CIC's guidance:
"Any period of employment when the applicant was engaged in full-time study will not be included in calculating the period of qualifying work experience (e.g., work experience gained through co-op work permits, off-campus work permits while a full-time student and on-campus work permits)."
http://www.cic.gc.ca/english/resources/tools/perm/econ/cec/work.asp
After you graduate, your OCWP authorizes you to work for 3 months; then if you apply for the PGWP before the end of that 3-month grace period, you are in implied status.
So - (1) your work IS authorized, and (2) you are no longer a full-time student.
Others have succeeded. But you have to make the case and provide the proof.
But as others have pointed out, if you want to avoid the issue, you can simply count your experience from the start date of your PGWP, and then you don't have to worry about it at all. Some people have the ability to wait and work the extra time, but not everyone does.