Off camera flash for a newbie

I am new to photography and have an Olympus Em5 mk1. I am undertaking a photography course and have to shoot a portrait for my next assignment.

I am looking to use an off camera flash but am not sure what to purchase that would work with my Olympus.

We also have a Nikon 5300 in the house so an off camera flash that would work with both would be great. If this isn't possible I understand.

Can someone recommend a flash I can purchase to help me achieve what I want?

Thank you

For underwater use? Sounds like dry land use... Yongnuo sells cheap radio triggers, and speedlights but I think the triggers are split by nikon or canon. You could always buy a $20 physical cable or set the speedlight in manual mode where it looks for a flash to fire, and use the flash on the camera as well.

No for normal use - I have been to my local camera exchange and bought one. Thanks for answering.