How can one?
The symbols available at http://dictionary.oed.com/public/help/Advanced/symbols.htm are "searchable special characters used in the OED."
The method to be used is explained at http://dictionary.oed.com/public/help/Advanced/exact.htm:
<<
1. Open the Advanced search page by clicking the button at the base of the screen.
2. Enter your search terms in the boxes in the usual way, except that if the word you are looking for contains an accented letter or other special character, enter its name instead of its plain letter equivalent, e.g. type caf#eacu# to find caf�.
>>
Accordingly, since the name of the per symbol is given as "per", one would enter #per# in a search field which one would get to from the main page at http://dictionary.oed.com if one had an OED on-line subscription.
Jim Allan
