I ran an 'impulse spectrum' test an O-port in an OM size guitar a while back. The guitar was one of a 'matched' pair in mahogany and Red spruce that I made as an experiment. The test is a fairly standard way of seeing what the guitar is capable of. You tap on the bridge and record the 'thump', in a more or less standard manner and setting, so that you can compare the effect of changes directly. This tells you what the guitar does, which may not be the same as what it sounds like.
What the O-port did was to drop the pitch of the 'main air' resonant peak in the output by 8 Hz, from 100 Hz (between G and G#) to 92 Hz, just below F#. The power of the resonance was also reduced, relative to other higher ones that did not seem to be affected as much, by about 20%. What surprised me, though, was what happened to the 'main top' resonance: that dropped in pitch by a similar amount (201 Hz to 192 Hz), but was cut down in amplitude by more than half. Normally the 'main top' peak in the output is much higher than the 'main air', and that was so without the O-port. Whit the port in, the 'top' peak is much lower then the 'air' peak.
This is much different from the changes that one sees with a normal 'tornavoz' which has straight sides that are either nearly cylindrical to somewhat conic. The usual tornavoz drops the 'air' pitch and power, often by a lot, but usually doesn't affect the 'top' mode nearly as much. The flare of the O-port seems to alter the air flow inside the box so that it works almost oppositely from the usual soundhole restriction.
Again, this is an objective test, and only on the one guitar. I would not expect the outcome to be very much different in kind on another guitar; I have not had the chance to check that out. It doesn't say what the changes will sound like; to paraphrase: "there's many a slip between ear and brain". Microphones don't have opinions; they just tell you what's there.
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