If the out-post O ring were leaking, I’d expect to see beer collecting around the base of the post and maybe running down the side of the keg. Rapid CO2 loss would be more symptomatic of leaky seal on the in post. I’d tear down the keg and pay particular attention to the dip tube and poppet as they’re more likely suspects. If you’re using a floating dip tube, my first guess would be the silicone tubing is too long, causing the inlet to be at surface level’