Creating tools that are meant to fit standard bit holders requires a very high precision.
To help you figuring out how precise is your printer (or your settings), or which flavour of that OMT² bit you like so much to take ...here are a few tests for you.
This is just a print test. To know more about the project, visit : http://www.thingiverse.com/thing:1381028
Tight (No tolerance)
This test is exactly 1/4" (or 6.35mm).
Depending on your printer's precision, it will probably be too tight to fit any bit holder
Basic (tolerance = 0.1mm)
This test is slightly smaller than 1/4". It's exactly 6.25mm.
It should be the right size to use with the OMT² handles, as well as standard bit holders.
Loose (tolerance = 0.2mm)
This test is smaller than 1/4". It's exactly 6.15mm.
In case the basic version is still too tight, this one is the way to go.
Print and try, there is no secrets here ^^
Let it cool down a couple of minutes before trying out, as the plastic will still be soft.
Tips and tricks :
It's easier to fix a bit too loose than too tight. For that, just put a tiny bit of Blu-Tack down the socket (the female part) and you should be good for hundreds of uses before having to change it.
This test is normally printed in less than 5mn.