Is there an "opposite" of glaucoma?

In a way, yes!: If the pressure of the eye is too low, the opposite of glaucoma where it is too high, damage and loss of sight may be associated. If the pressure drops due to injury, say, then the low pressure is an effect of the loss of function rather than being the cause of the loss of function. This is called phthisis bulbi (pronounced "ty-sis bulb-eye". Hopefully i made the difference clear!

Hypotony: Usually when too much fluid gets out of the eye like post surgical, post trauma, or ciliary body shutdown due to meds, inflammation, etc.