How many OPPO users participated in events which were held around 12 AM on 1st May,2016?
OPPO refers to phone_brand = 'OPPO'; around 12 AM on 1st May, 2016 refers to timestamp is '2016-05-01'
SELECT COUNT(T1.device_id) FROM events AS T1 INNER JOIN phone_brand_device_model2 AS T2 ON T1.device_id = T2.device_id WHERE T2.phone_brand = 'OPPO' AND STRFTIME('%Y-%m-%d', T1.`timestamp`) = '2016-05-01'