Arrow - Spoilers galore

Particularly Oliver Queen, who spent years being trained by DeathStroke/Slade, along with Sara, at least for the first or so they both on the island before they were separated when freighter flooded, plus being trained as Argus assassin in Hong Kong by Amanda Weller when he briefly fled the island 3 years after he and his father supposedly died on that luxury cruise accident, which clearly we know now was no accident. Only thing that seems bit odd is that Oliver never had any run-ins with Sara was picked up and trained as an assassin by Nyssa Al-Ghul's League of Assassins before his first rescue. Working with a clandestine, covert spy agency like Argus it's reasonable to at least assume Oliver heard of them or at least know about rumors surrounding Ra al-Ghul or the League of Assassins or that one of their best assassins was his ex-girlfriend.
Off-topic but one thing I wish they'd consider trying in a future Avengers movie is touch on Wolverine
s history with Nick Fury/S.H.I.E.L.D and there is enough comic material to tie it in since Nick Fury's S.H.I.E.L.D worked black-ops missions with Wolverine's Alpha Flight or with him, Deadpool, Sabertooth, and Lady Deathstrike in Department H in the late 60's/early 70's before Logan quit and joined the X-Men and of course, Wolverine didn't exactly have a good relationship with S.H.I.E.L.D or Nick Fury, but he's one Marvel Universe character that would create an interesting Avengers sub-plot if they decided to explore it in a future movie that's different from Thor, Iron Man, or Black Widow.
FWIW, I've said this before but this show took a big risk bringing in a complicated, complex character like Ra al-Ghul and intertwining into its chronology, and it's worked, Ra's not your typical amoral psychopath like the Joker or some vengeful villain like Two-Face who has dissociative-personality disorder, he and League of Shadows fit in lot more as villains in Batman's universe, because again at least there's more signs of mutual respect, in the comics, Al-Ghul never shied away from saying how much he respected Batman/Bruce Wayne, or that he was the best assassin he ever trained before he became Batman, or that his daughter, Talia Al-Ghul, love for him wasn't genuine. It sort of like the General Zod/Superman dichotomy where both men, initially, have a shared mutual respect, but become enemies by either circumstances or that one's side's beliefs become too extreme for other to tolerate.