To paraphase a Mopar expert from another forum:
The OSAC delays vacuum to the distributor vacuum advance pod by 7 to 17 seconds (depending on the exact valve in your vehicle). It was designed with one purpose in mind: To get the vehicle configuration past the ludicrously unrealistic Federal Emission Compliance Certification tests. Removing it allows the vacuum advance to work normally, which greatly improves driveability and mileage with zero negative side effects, and even if you have to pass an emission test of some description, you'll still pass just fine without the OSAC valve.
Last edited by CatFish; December 15th 2017 11:04 am. Reason: credit due