Cut then oil. I don't like getting oil on my kit - it can be transferred onto other stuff you don't want to oil and give you painting/finishing problems later on.
It depend's how much control you have over the job and how much you are doing yourself. If other crafts people will be working around you, getting hand marks and rubbish on the tops, a sealer coat could help. But you probably have a lot of work to do anyway, sanding joints and doing cut outs, so it won't be worth going mad with the oil at the early stages. Get plenty of thin coats on before the kitchen is working, especially round the sink. In the early stages of oiling it can bloom with water. Remember oily rags can self combust.
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