Coincidentally, I've just been working on a schematic which requires an op-amp bistable. I chose the comparator, rather than an IC or discrete bistable, because the trigger voltage can be set fairly accurately.
Here's the circuit, but using the LM358 and changed so I can post it here. The original design uses an LM393 comparator and is part of a bigger design. If Schottky diodes are used, the forward voltage can be ignored for most applications and the bistable will fire when the inputs are above V2. Using high values for R1 and R2, reduces the errors from the diode forward voltage, but increases the errors from the diode leakage and op-amp bias currents. Adding resistors with the same values as R1 and R2 in series with the non-inverting inputs will minimise the latter.
R3 and C1 form a power on reset circuit, which ensures it always starts up in the same state when power is first applied.