Beat your slave

If you use an old optical trigger slave with your in camera flash set to anything but manual, you will have problems. The pre-flash used my most cameras to meter for flash exposure trips fractions of a second before the shutter opens for the exposure. This often doesn’t give most flash units time to recoup and fire again. If you put you Olympus inbuilt flash to minim manual power it only fires when making the exposure.
You can use this knowledge to buy up all the old optical slaves super cheap.