My understanding is that you can resubmit and choose "No Qualifiers." However, the straight grade will certainly be lower. PSA's brochure (the link and an excerpt are below) happens to use your example -- an 8 (OC) being graded as a straight 6.
Basically, I think when a card has all the attributes of a specific grade except for centering then it will be given that grade and an OC qualifier unless you choose "No Qualifiers" ... in which case it will get a lower straight grade.
No Qualifiers:
Cards having significant flaws will receive “qualified” grades (see definitions online at PSAcard.com.) If you select this box and your card is deterimined to have a qualifier, your card will be graded with a lower straight grade.
For example, what would be graded an ‘8 OC’ might be graded a straight ‘6.’
*Please note: Some qualifiers may not be removed.
http://www.psacard.com/Content/pdf/P...-To-Submit.pdf