that suggests that exchange orderbooks are only a small fraction of the latent supply of bitcoins in the market, since Saylor is buying OTC.

I'd say if one was genuinely interested in my somewhat lighthearted analysis of Strategy's actions, this is a pretty good conclusion.

I am curious how one would discover if there is some trend between what we see as "the price" (my impression is that "the price" is usually taken from an average of various exchanges' APIs -- or just one exchange if you are poor) and the trades that happen OTC.

Do the prices at which OTC trades are confirmed lead the exchange prices? Do they lag? Do they fluctuate somewhat independently?

If the OTC market has so much more volume than exchanges, do they almost operate as separate markets? Clearly there is some connection, else all the exchanges would be constantly getting wiped out.