I think that Brock Osweiler should be on your fantasy football team because his ratings are in the solid 80% range -- this means he is more than capable and probably up and coming. He also would be a good person to have on your team because he is not as sought after as someone like Tom Brady, so you can ride his wave of success as he progresses -- he's only 25.
Brock Osweiler, as a replacement for an ailing Peyton Manning, is only adequate. His chemistry with his receivers is not on par with where it should be as demonstrated by the plague of drops both Saunders and Thomas have had over Osweiler's tenure. If you are working in daily leagues, Osweiler isn't the QB starter to have. Other cheap options such as Tyrod Taylor or Blake Bortles are more likely to put up fantasy numbers depending on the matchup. If you are looking for a new QB in your season-long league, most likely Taylor and Bortles are still available. Osweiler's run of wins has depended more on the influence of one of the top defenses in the league than his individual play.
Brock Osweiler is not a good player to have on your fantasy football team because he is not a starter. As the backup quarterback for Peyton Manning, he did not get playing time until the tenth week of the season. Once he did get playing time, he does not get a very high number of fantasy points. There are many other better quarterbacks for the 2015 season. However, if he is picked up by another team in the 2016 season and is a starter, he may be a good choice.
He's not a good player to have your fantasy team. There's no guarantee how much longer he will even be a starter. When Manning comes back, Osweiler could very easily be back on the bench. I don't think this is the situation you want to be in with the playoffs approaching.