Windsor is outside the TfL zones, so no, you cannot use it to travel to Windsor & Eton Central. Simply buy rail tickets on the day you plan to travel from Paddington station; there is no advantage when buying the tickets in advance.
Here's the map of where you can use Oyster: …tfl.gov.uk/london-rail-and-tube-services-ma…
Trains also run from London Waterloo Station to Windsor and Eton Riverside Station if that would be more a more convenient departure station for you. Same reduced price offer for Castle tickets.
Perfect, thanks for the info!