sand them lightly to remove all the marks and stains on them apply one coat of varnish let it dry then with a very fine sand paper sand them again it will produce a better finish
you could thin out the varnish a little for the first coat as this seals the wood, lightly sand with 240 grit sandpaper, wipe off the dust with a tack cloth, varnish again, depending on how many coats of varnish you use, lightly de-nib with 240 in between coats
Yes, get clear as this is the point of getting oak, when you varnish it, it brings out the knots and grain in a rich wood colour. Clear varnish soaks in and it will naturally go a rich colour