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From EU3 Wiki
A One-province Minor or OPM is a common term for a small nation that starts with only a single province. AI OPMs tend to be quickly annexed by larger neighbours. However, many players like to start as an OPM for extra challenge.
In 1399, most initial OPMs are in Italy and Germany, often protected by the Holy Roman Emperor. There are also a few in other parts of Europe and in Asia. One advantage of OPMs is that their small size means a low Country size research modifier. This enables them to research quickly and, despite a small income, keep in the same technology levels as other, larger countries.