Provide the total number of the male users that use OPPO as their phone brand.
male refers to gender = 'Male';
SELECT COUNT(T1.device_id) 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' AND T1.gender = 'M'