the majority of oil leaks arent life threatening apart from the fact u have to keep replacing the lost oil..... oil leak could be anything.... u need to dip a bit of white paper into the puddle and find out what colour it is.... also shuv your camera under the car and take a pic of where the oil is coming from..... try and establish if its from the engine.... and if it is, things like is it coming from the front or rear, or either side.....also dont think just coz its got 30000kms on it things wont go wrong... iv seen brand new cars leak oil, or cars with 10000 blow a motor....
as for the missfire.... mate this could be any number of things....start off checking all the usual things.... check your spark plugs, your coils, all your electrical connections for the injectors, sensors and earth and get bak to us.