any flash unit may be used with Olympus cameras as long as the trigger voltage is below 24V
most old flash units from the 1970's and earlier tend to have high trigger voltages which may disintegrate your camera's circuitry!
older flash units for Minolta/Sony which use their proprietary pin layout will require an adapter to fire from the Olympus hotshoe
manual flash output set from the camera
requires a built-in or bundled flash to be activated (other external flashes cannot have their manual output controlled by camera as this may conflict with the flash unit's settings)
in the Live Super Control Panel, set the flash mode to a manual output value (press INFO button then change with arrow buttons)
manual flash output set from the flash unit
set flash unit to Manual or FP Manual
set the flash unit output via the flash unit's output setting
the camera's flash mode items will not have a manual output level available, you must use one of the other Olympus OM-D flash modes