There seems to be two parts to the OpenHashSet's API -- functions one would normally think exist for a Set, and functions that are needed to implement the OpenHashMaps. Do you think there might be other users of these seemingly internals APIs? If not, you might consider making these private[collection] or, even more dramatically, splitting up this class into two. There seems to be very little shared functionality between the two APIs:
<table>
<tr><th>Public</th><th>Shared</th><th>Internal</th></tr>
<tr><td>contains</td><td>size</td><td>capacity</td></tr>
<tr><td>add</td><td></td><td>addWithoutResize</td></tr>
<tr><td></td><td></td><td>rehashIfNeeded</td></tr>
<tr><td></td><td></td><td>getPos / getValue / nextPos</td></tr>
</table>
the ``nextPos`` thing is a poor-man's iterator, a public API should implement Iterable.