Write SQL query to solve given problem: What is the age group of most OPPO users?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT T.`group` FROM ( SELECT T1.`group`, COUNT(T1.`group`) AS num FROM gender_age AS T1 INNER JOIN phone_brand_device_model2 AS T2 ON T1.device_id = T2.device_id WHERE T2.phone_brand = 'OPPO' GROUP BY T1.`group` ) AS T ORDER BY T.num DESC LIMIT 1