Ow5 is a gag aimed at the craziness of where we are in O scale 2 rail re standards. Might have originated in the UK where we would see 1:43.5 (UK) , 1:45 (EU) and 1:48 (NA) scale layouts at shows, all running on 1.25" gauge "O scale" track.
I first heard it from Mike at Gilmaur (I think) 20 years ago as P:48 started to grow over there.