The two of you knew each other through work. Superhero work. Being in the Justice League caused both you and Oliver to constantly be in close proximity and the frequent interactions between the two of you to become close. Really close.

"I can't let you go with me!" Oliver snapped, grabbing your arm. Deathstroke was a powerful mercenary and Oliver knew between him and Slade, only one of them would come back. Adding you to the mix was too dangerous.