Subaru Impreza starting

I go to start my subi first thing in the morning and it takes at least 10 times to start sometimes i flatten the battery.some days it will start quicker if its a sunny day.the battery is new.i was told to turn the key on til i hear the petrol pump kick in then start the ignition.still its hard to start.some mechanic told me sounds like its an oxygen censor.after i get the car started it will start normally all day.it must be a common problem??? its annoying.ive booked it in to get a diagnosis test done as the check engine light stays on.