How to optimized mysql query having large dataset
First, if email_id and delivery_id together are a unique key, please add a primary -composite- key on both tables for (email_id, delivery_id)
Second, the concat is not necessary and will prevent the previous key from being used. Try:

SELECT ol.* 
FROM open_log ol 
JOIN sent_log sl 
ON (ol.email_id, ol.delivery_id) = (sl.email_id, sl.delivery_id)