@thibaud I think just to keep things simpler, please check if just simple mautic API works using curl.
By OAuth, I guess you mean the Mautic's "OAuth2" i.e the one described in https://www.mautic.org/blog/developer/how-to-use-the-mautic-rest-api ? I have no clue how that works...
But for our own mautic automation, we first enable the API like this:
Then, we use http basic auth. We then created a separate user called mauticbot (added straight inside mautic). We then use https://github.com/vdavid/node-mautic to automate. That module just uses the account username and password and no API token.