osr relay

the OSR is an oil surge relay, similar to the buchholz surge flap usually placed on the oil pipe between the OLTC and its oil tank to respond to rushes of oil from the former indicative of an internal fault. In the application you describe, it is used to protect against faults in the cable termination box that would result in a rush of oil to the small conservator. Such a fault should open both transformer breakers (internal fault), and if the OSR device is operating for faults external to the transformer it could be that the fault current is sufficiently high to cause some heating in the cable termination box or the termination is on the verge of failing.