Oscar Guzmán is a character in Grand Theft Auto V. Guzmán is a Mexican cartel boss and gun runner who is friends with Trevor Phillips, and pays him for the cargo delivered in the mission "Nervous Ron".
If Trevor buys the McKenzie Airfield in Grapeseed, he can drop packages of guns off for Guzmán's buyers, and will be paid for it. He tells Trevor that he has been north for around five years on-and-off, because there was too much competition south of the border in Mexico.
He's an independent gun runner that keeps a moderate profile in the crime scene, as he's avoiding making enemies with the most powerful Mexican gangs in town like the Vagos or the Madrazo Cartel, yet he's still competing with the Varrios Los Aztecas and the Marabunta Grande.