How would you write it in a story.
'oh shit' moment.
'Oh shit' moment.
'Oh Shit' moment
or some other variant?
A quick Google search has shown all those variants in common use. I'm veering towards 'Oh Shit' moment, purely because it looks lop-sided when only the 'Oh' is capitalised.
Yet another not 2 by 4 worthy issue ;-)
AJ