Warning: spoilers ahead for Episode 22 of Arrow Season 6, called "The Ties That Bind."
The penultimate episode of Arrow Season 6 finally unleashed Ricardo Diaz with all his resources on the vigilantes of Star City. While Diaz himself is hardly the most daunting bad guy ever to go up against Team Arrow, he has managed to buy, barter, and murder his way into a position of ultimate power in the city, and the good guys were on the run almost as soon as he decided the time had come to kill them off. In fact, the only reason Diaz was unable to kill the entire Team Arrow -- former and current members alike -- was that Anatoly had enough loyalty to Oliver to give him a heads up that Diaz was going to have him and his family murdered.
Unfortunately, he only managed to give Oliver a few seconds of a heads up, but it was enough for Oliver to warn his pals and prevent his family from being killed. Diaz had too many connections for them to simply retreat to the Arrow bunker or draw on their regular resources. Even Lyla and her A.R.G.U.S. connection could only go so far. None of the key players in Star City seemed to have the reach to possibly outflank Diaz. The end of the episode revealed that Oliver is willing to reach out to somebody unexpected to step in and save the day, and that person is none other than FBI Agent Samanda Watson.
Samanda Watson has been a thorn in Oliver's side from almost the very beginning of the season. She arrived on the scene to investigate Oliver, believing him to be the Green Arrow. While she got so far as to arrest him and interrogate those around him, she was never able to amass enough solid evidence to nail him for his crimes. The previous episode had Oliver on the verge of going to prison after being convicted by a jury purchased by Diaz, but Human Target was able to step in to set Oliver free by posing as the judge of the trial. After the trial, it seemed like Oliver's troubles with the law and involvement with Samanda Watson were done. "The Ties That Bind" points toward Oliver ending up behind bars as the Green Arrow after all.
When Oliver went to Samanda Watson for help, she seemed amenable, but only if he admitted that he's the Green Arrow. Oliver gamely did so, landing himself firmly on the hook with the FBI for the sake of saving Star City. Given that the finale episode is called "Life Sentence," we may want to prepare ourselves to see Oliver arrested before the season is over. Good for Oliver that he can prioritize the safety of his city over his own freedom; not so good for Oliver if he winds up in jail with a bunch of people that he helped lock up.
That said, a lot can happen in an Arrow season finale, and the trailer for "Life Sentence" doesn't show Samanda Watson or Oliver in custody, so we can't say for sure what will actually go down. Take a look: