Op-amps are normally not used in an open-loop configuration except when being used as comparators and/or oscillators due to its extremely high open-loop gain. When used as a comparator, an op-amps compares two voltages or currents and switches its outputs to indicate which is larger.
A standard op-amp used as a comparator, is shown below.
Vout = VDD when V1 > V2
Vout = VSS when V1 < V2
When the non-inverting voltage input (V+) is higher than the inverting input (V-), the high gain of the op-amp causes it to output the most positive voltage it can possibly be (positive rail). When the non-inverting input (V+ ) drops below the inverting input (V- ), the op-amp outputs the most negative voltage it can possible be (negative rail).