The eruptive history of Opal Pool is one of those "half full, half empty" propositions: in most recent years, it has been active, but the number of (reported) eruptions per year has been small. For example, in 2014, reported eruptions consisted mainly of a series of closely spaced events in November, but no eruptions have been reported since then. Observer bias may be a factor here: Midway Geyser Basin is overrun for most of the open season by crowds clamoring to see Grand Prismatic Spring, and experienced gazers (at least the ones who report their observations), knowing that actual geysers at this basin are relatively few, tend to minimize their time there to avoid being overrun for no good reason. In any event, there is no doubt that Opal Pool's eruptive cycles are infrequent.
When it does erupt, it's the second most powerful eruptive feature at Midway, behind huge but rarely seen Excelsior Geyser, briefly active during 1985 but not since then (nor for many years before). Its very brief (often a single burst), dome-shaped eruptions have reached heights as great as 70 feet, although in its most active days, in 1947 when it was first observed, 50-footers were the norm.
Not to be confused with "Black Opal Pool," another infrequent but powerful erupter (and beautiful pool) in Biscuit Basin.