Well, as I understand it, stations need to pay Nielsen to get rated and they have chosen to not do that. I am pretty sure it has nothing to do with not getting rated by Nielsen because viewership is so low, but what do I know, I am just posting sourced information while you have the infallible treasure trove of knowledge that is your crossfit brain.
Here's an article about CBS's contract with Nielsen is expiring.
https://www.forbes.com/sites/bradadgate/2019/01/07/can-cbs-go-without-nielsen/#79f2878d4b57 They had paid Nielsen $100M to do ratings stuff for them:
"It has been estimated the previous contract between the two parties had a value exceeding $100 million. Besides the broadcast network, the CBS contract includes their 28 owned and operated stations, primarily in major TV markets. CBS claims that Nielsen’s audience measurement is inadequate in today’s multi-platform environment and that Nielsen’s asking price is too high.
Without a contract, CBS will not have access to any current or past audience data. CBS will, for now at least, rely on audience data from Comscore and other sources for ratings. Comscore uses return path data from several satellite companies and cable operators including AT&T/DirecTV, Dish, Cox and Charter. Comscore represents the biggest challenge to Nielsen’s dominance since Arbitron (now owned by Nielsen) left local TV measurement in 1993.
Virtually all prominent ad agencies and television networks/stations are Comscore clients."
It seems OAN, like a number of other companies, has decided to go with Comscore instead of Nielsen.