For the simple syrup: In a small saucepan, add the sugar and 1 1/4 cups water over medium heat. Stir to combine. Let the mixture simmer for 2 to 3 minutes, until all the sugar is dissolved. Remove from the heat and let cool (can be made a day in advance and stored in the refrigerator for 5 days).
For the drink: On a grill pan over high heat, grill the orange flesh-side down until charred and caramelized. Set aside and cool slightly.
Squeeze the lemons and limes into a pitcher. Add 1 1/4 cups of the simple syrup. Mix in the tequila and orange liqueur. Stir with a wooden spoon. Squeeze in the grilled orange and stir to combine.
Fill glasses three-quarters of the way with ice. Pour the margaritas into the glasses and serve.