How to make washed oak?

Don't know exactly what you mean by 'washed oak' but I used to make oak furniture with a very easy, simple and effective finish which was:

Plenty of work with a wire brush (in a drill) to open the grain.

Oil based gloss household paint in white/cream diluted about 50-50 with white spirit was slopped liberally by brush all over the piece. Immediately, before any drying, rub off all the excess paint you can with plenty of clean cotton rags.

Allow to dry. Depending on the coarseness of the wire brush and the dilution of the paint you will have an effect from a fine shiny limed oak finish to a rough weathered driftwood look. The wood will be sealed, at least partially. You can buff up with coloured wax if you wish.

Tim w