I have OSM on my garmin Oregon and they are perfect. I downloaded the file from the OSM web site noted here. For example, I clicked on "download" under the word Queensland and saved it to my C drive. The file had a name QLD.img and it was a winzip file and then the filename was something like 610501.img
I copied this file to my gps which came up as the F drive and put it on the directory "Garmin". I have also put some other state maps on the removeable chip which is G:Garmin and they work really whell too.
To check if the maps are ok, I go to "find a geocache" and select a cache I know is in the area I'm trying to check. If I look at a zoomed out level, not much is visible but when I zoom in, all the streets come up and i can see it's all ok.
Have a go and tell us what you see