Final answer:
An ohmmeter will display 'OL' when test leads are placed across an open circuit, indicating that the resistance is too high to measure, effectively infinite, and the circuit is open.
Step-by-step explanation:
When measuring circuit resistance with a Digital Multimeter (DMM), if you place the test leads across an open circuit, the ohmmeter will display 'OL' or a similar indication, which stands for 'overload' or 'open loop'. This reading signifies that there is no continuity in the circuit and the resistance is extremely high, effectively infinite. The ohmmeter cannot measure an infinite resistance, so it gives a reading that indicates the circuit is open.