In a mixing bowl, combine olive oil, soy sauce, red wine vinegar, garlic, black pepper, salt, and oregano. Mix well.

Place the flat iron steak in a resealable plastic bag and pour the marinade over it. Seal the bag and refrigerate for at least 1 hour, preferably overnight.

In a bowl, combine chopped parsley, olive oil, red wine vinegar, garlic, red pepper flakes, salt, and black pepper. Mix until well combined.

Set aside to let the flavors meld while the steak marinates.

Preheat the grill to high heat.

Remove the steak from the marinade and discard the marinade. Grill the steak for about 4-5 minutes on each side for medium-rare, or until desired doneness is reached.

Use a meat thermometer to check for an internal temperature of 130°F (54°C) for medium-rare.

Remove the steak from the grill and let it rest for 5 minutes before slicing.

Slice against the grain and serve with chimichurri sauce on the side.