Does restoring my iPad revert it to the originally installed OS?

Restoring your iPad to factory settings will cause a fresh copy of the most recent compatible version of iOS to be re-installed. For the iPad2, this will be iOS 9.3.5 (WiFi Only models) or iOS 9.3.6 (WiFi+Cellular).

However, before restoring your iPad, you need to consider that the iPad2 is a 32-bit device - and along with other 32-bit architecture devices, is obsolete. Compatible Apps for 32-bit devices are no longer available for download from the App Store, all having been retired and removed.

As older versions of Apps have been deleted from the Apple App Store, you’ll not be able to (re)download or install them - which may severely limit the usefulness and utility of your iPad. As such, if you want or need to access to available Apps, all requiring more recent versions of iOS than your iPad is capable, you may need to consider replacement of your iPad with a newer or current model.

Of current models, even the entry-level iPad9 has specifications and capabilities that will massively outperform you current iPad - and is compatible with the vast majority of current Apps. Other models are more powerful still.