Yes, there is a sheet feeder. But OKI calls it a "multi-purpose tray". That's why you can't find anything on the web when you search.
As Beat.bickel correctly explains, you have to fold down the top (wide) front panel. This gives you access to the multi-purpose tray and the sheet feeder.

Hello
Yes, fold down the lid at the front. This way you have a drawer (A4 for us) and a sheet feeder (A3 for us).
Very good device with a fast scanner.