OMD time exposure - what am I doing wrong?

I decided to try a test of a time exposure using just moonlight. See below - my backyard. The exposure is about right I think, but the quality of the picture is terrible - there is just no detail at all, it's just horribly smeared.

Camera on tripod with cable release. IS turned off on lens and camera. I focused manually first with focus assist (but it's f7 anyway, so depth of field should be good enough.) ISO 200, 5 minutes on the bulb setting. I tried several times with no improvement:



Cropped center of shot:



Does anyone have an idea what I am doing wrong? Surely it's supposed to look sharper than this?