It depends on how realistic a game you're running.
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In real life, when you pour oil on a sword, the oil basically just runs down the sword, and only a very thin coating will remain on the blade.  This coating will either evaporate before you can do anything useful with it, or burn for a moment or two before burning away.  In the best case, you get a bunch of burning oil on your hands.

Now, you might be running a more cinematic game, where you want people to be able to pour oil onto their swords and have it be an effective tactic.  If that's the case, then you would probably use the same rules that a torch does, and have the sword do 1 point of fire damage in addition to its normal damage.  How long this buff will last depends entirely on how cinematic the game is, but it probably shouldn't last more than a few rounds in anything but the most ridiculous of game worlds.

Burning weapons in real life are almost exclusively based on projected fire, or blunt impact of a flaming object.  It's not really possible to light a sharp blade on fire without either providing a continual source of flame (like a flamethrower), or using a material that is inherently flammable, which would screw up the weapon's efficacy as a blade.