OJS can do this without modifications. If you don't want users to require registration to read abstracts, look up the settings in page 4 of the journal's Setup. Depending on your version of OJS, you should see a setting titled "Users must be registered and log in to view open access content." Make sure this setting is disabled.
This way, you can configure your journal for subscriptions, but abstracts will be open access and registration won't be required to view them.
Public Knowledge Project Team
Don't miss the First International PKP Scholarly Publishing Conference
July 11 - 13, 2007, Vancouver, BC, Canada