Slippery decking usually occurs when timber decking boards develop mould, algae or other biological growth, usually identifiable by green or black staining in the wood. This normally happens on bare wood decking that hasn't been treated or previously oiled decking that hasn't been maintained for a number of years.
Remove as much of the surface growth as possible with a stiff broom, brush or scraper. When done, treat the decking with a decking cleaner and one that is designed to kill off mould and algae spores in the wood. Heavily soiled decking may require a second treatment. Then apply an exterior wood preservative to help prevent future biological growth before applying a decking oil. For additional anti-slip properties, use a slip resistant decking oil such as Osmo Anti Slip Decking Oil (430) which is especially good for decking steps.
To stop decking from becoming slippery again, keep well maintained by regularly sweeping off any leaves or other organic matter, cleaning from time to time with a dedicated decking cleaner and then re-oiling as and when necessary.