INDEX(meta_key), this may be faster:
WHERE meta_key LIKE 'ddw_%_op1' AND meta_key RLIKE '^ddw_[[:digit:]]+_op1$'
LIKE (without a leading wildcard) can use the index. Then the
RLIKE will verify that only digits are in the middle (without using the index).
Since this smells like a WP schema, here are some more tips on
wp_postmeta. (The tips won't speed up your ddw query, but may help others.)