Should I repot?
 

I bought this cute oncidium at a fair yesterday. The fellow told me not to repot while it's blooming.

Would you agree?

I would agree, BUT...

I suggest being very delicate with your watering, that is, only moisten the potting medium slightly, rather than soaking it, until you are ready to repot.

The apparent density and age of that potting medium suggests that it could be easily waterlogged, suffocating the roots.

The best time to repot them is when the new growth has roots.