Depending on the type of timber used for the deck and the amount of exposure to the elements, the type of oil and frequency of oiling will vary.
Generally it is recommended you re-oil your deck at least once a year with a suitable decking oil.

Personally, I prefer to use raw linseed oil thinned with mineral turpentine.
I have found that linseed oil actually lasts quite a while and leaves a nice finish and as a bonus is cheaper than other expensive timber treatments.
If you would like to add colour to the oil just add some wood stain to the mix.

If the timber deck hasn't been maintained for some time and has greyed or blackened I recommend washing it with a high pressure washer and using a deck reviving solution. After washing down allow deck to dry completely prior to oiling.

Tip: Using a roller apply two light coats rather than one thick coat. Allow some drying time in between coats.

Note: Follow all products safety instructions and procedures for handling and working with in a safe manner.

I hope these tips help and if you have any comments or questions please feel free to post.