I want to connect an application (Oscar) to Google Fit to record my CPAP results.
Oscar is an application and not a mobile or web app, so I would have to push the data manually. It doesn't seem to be a difficult job, but I'm wondering if it's allowed. I can't see anything that forbids desktop applications or CLIs to interact with Google Fit, but I can't see anything that allows it either. The documentation only talks about websites and apps.
There is no reason in general why a command-line application cannot write data to fit: it's ultimately all just data.
However, the fact you are trying to write data about a medical device means that you cannot use Fit. From the terms of use:
Google does not intend Google Fit to be a medical device. You may not use Google Fit in connection with any product or service that may qualify as a medical device pursuant to Section 201(h) of the Federal Food Drug & Cosmetic (FD&C) Act.