Changed oil and filter and my 2003 wrx today. Tryed start after about hour or so and was like bat was dead. Jump leads on fired fine. Drove for around 30mins stopped and started fine. Again hour or so later car struggled start. Any ideas guy? Ive read about disconnecting crank sensor etc.

 

Likely to be coincidence that it happened after an oil change, in my experience. Not sure of the cause though.

 

Did you have the door open whilst doing the oil change? Interior light/head unit maybe drained it?

Go for a long drive with sidelights on, or trickle charge it over night.