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Mild dish soap, like Dawn, is a great degreaser. Address the oily stain as soon as you notice it; the longer it sits on the surface of your deck, the more difficult it will be to remove. Use a soft bristled brush and soapy water to scrub at the stain. Rinse the soap away thoroughly with hot water. If the oil based stain has set and mild dish soap isn't removing it, try an oil stain remover product like OSR or Pour-N-Restore. You can buy these products at home improvement stores. Water spots, also known as tannin stains, are normal occurrences on wood. Over time, you will likely start to notice some on your deck. Sweep the deck free of all dirt and debris. Make sure the surface is completely dry and then apply a deck brightener product that contains oxalic acid on the surface of your deck. Check your particular product for any additional directions. Deck brightener products can be purchased at home improvement stores. Deck brightener products will also work on these tougher stains. Sweep the deck and apply the product directly to the stain. Saturate the area and allow it to sit on the stain for about 15 minutes. Rinse the spot thoroughly with a water hose.
Use dish soap and hot water on oil based stains. Use a deck brighter than contains oxalic acid on tannin stains. Use oxalic acid on rust stains and other stubborn spots.