Sources close to the production tell us the "Teen Mom 3" star, and fan fave, has officially joined the 'OG' cast, and she's already shooting scenes. As we told you, Farrah was 86'd and claimed, on her way out the door, MTV execs shamed her for doing porn.

We're told 'TM' producers recently pitched Mackenzie the idea of joining 'OG.' They were already shooting a 2-hour special with her about her mom's cancer battle. She was diagnosed earlier this year with stage 4 brain cancer.

We're told Mackenzie agreed to step in for Farrah and she's been filming for more than a week now.

You'll recall ... Mackenzie was part of the 'TM3' franchise before getting canceled after one season in 2013. She was very popular then, and fans have wanted to see her come back.

While Farrah's off fighting Viacom and MTV in court ... it looks like 'Teen Mom'-sters are getting exactly what they wanted.