A bottle of engine oil has leaked and soaked my back car seat, how can I fix this?

The way I have cleaned smaller patches of oil succesfully by
1. Scrape off as much oil from the surface as possible with a knife.
2.Dab the area with a towel, to soak up more oil
3.Wet a clean towel with washing up liquid, and dab on to the stain area.
4.Rinse with another towel wetted with warm water to rinse by dabbing again.
5.Repeat 3 & 4 until clean.
6.I have finished off with a wet cleaning vacuum such as a Vax

If the stain has become dryed, spray some WD40 to rewet it, scrub with a toothbrush if really solid & repeat cleaning cycle above.
I believe using a steam cleaner can work, but I have no expeience of this.

If you Google "clean oil from upholstery", the Wikihow sites are useful
How to Steam Clean Auto Upholstery eHow might be good too.

Hope this helps.