Lack of built-in flash on OM-D annoying or frustrating?

Just carry it around, the size of it is extremely small. Alternatively just keep it on and only flip it up when you need it. Most of the time it sits on mine and I use it when I feel I need to but otherwise it's just there. When I don't need it I just put it in its supplied pouch and put it in my bag or pocket and use whatever other accessory I may need. Not having it built in and placing it even higher from the lens means it helps to reduce red eye which is a problem the closer the light is to the lens. The ideal position is exactly where it is higher than eye level so it makes sense for position but because of the requirement for the built in EVF and 5 axis IBIS there lays the other issue. The final technical reason is because the accessory port is also used for other attachments like the SEMA-1 or MAL-1. So you can see because of the customization available and features they put in, the flash had to be done in a way that wouldn't be seen as a negative to image quality so they removed it and included it as an accessory.