The flash that I use with my OM cameras is a Sunpak DC3 Flash. I am not sure what the original manufacture date is on the flash, but the user manual has a purchase date penciled in it of 1971.
The flash unit mounts to the hotshot on the OM camera, though it is not hotshot operated like the Olympus flash units are. This one has a small cable that attaches to the flash synch port on the right side of the lens bayonet (if you are looking at the front of the camera). It operates in X mode only. A perk of the cable operation is, that if you are steady with your hands, you can hold the flash off the camera hot shot and use it as a bounce flash, though this is very akward. I tend to focus the shot first, then pop the flash off and then take the picture instead of trying to focus while holding the flash.
The flash has a distance, ISO and F-stop chart on its side for easy reference.
The hard data on the flash is as follows. It weighs 4oz and it measures 3 1/4″x2 1/4″x1″. It has a guide number of 30 using ISO100 film. It has a light angle of 65 degrees (about 35mm lens coverage). Its color temperature is 5,500k. It has a flash duration of 1/1000s. It has a flash recycle time of 6-8 seconds on fresh batteries (2 AA) and it has a battery life of approximately 200 shots.