COOK potatoes in boiling water 20 min. or until tender; drain. Place in medium bowl; mash until smooth. Refrigerate 1 hour or until chilled.
MEANWHILE, heat 2 tsp. oil in medium skillet on medium heat. Add peppers, onions, garlic and oregano; cook 5 min., stirring occasionally. Remove from heat. Stir in tuna, cheese and 2 Tbsp. olives.
HEAT remaining oil in medium saucepan to 375ºF. Using lightly floured hands, shape potato mixture into 8 (3-inch) balls, using about 1/3 cup potato mixture for each. Make large indentation in center of each; fill with about 1/4 cup tuna mixture, pressing tuna mixture into center of ball to completely enclose filling. Re-roll into balls, if necessary. Beat egg in small bowl; place flour in separate bowl. Dip potato balls, 1 at a time, in egg, then roll in flour until evenly coated.
ADD potato balls, 2 at a time, to hot oil; cook 5 to 6 min. or until golden brown. Drain on paper towels. Mix remaining olives and mayo. Serve with the warm potato balls.