Windows 95 OSR2 IIRC. If you bought Plus! I believe that upgraded you to OSR2 but there was an independent update and OEM build. I got a PC in 1997 with two USB ports but the documentation said they wouldn't work until a later Windows 95 release.

By some means I got OSR2 and the USB ports showed up in the Device Manager but it I had to wait a few years before I even owned a USB device to use with them.