When installing OPIM using typical default settings the file association for all DGN files is changed from Microstation to opening with OPIM as a default program.
How to set file association when installing OPIM ?
In order to stop from binding the DGN or other extension to OPIM, select “Custom” on the Setup Type Dialog:
And on the next dialog select “File Association” and pick the last option “Entire feature will be unavailable”
Installation - AutoPIPE