I have a light oak sideboard which now has a water mark ring caused by a vase of flowers very slowly leaking out of the bottom of the vase.

Any ideas how to get rid of this ring? I have tried soapy water but to no effect.

The surface is not varnished, merely oiled.

If the wood was oiled and not varnished the water has gotten under the oil. Before you try anything drastic try rubbing the spot with a terricloth towel and cigarette or cigar ash. It will act as a very fine abrasive.