I recommend Padam, Padam for a Older man/Younger woman relationship.
Also, one of the pairings of A gentleman's dignity is like that too, and there's also a Older female/Younger man relationship there :)
In My queen the female is older too n_n
Age gap is kind of thing so I have lots. I particularly i recommend Coffee prince and Sapuri and Whats up fox. since you have already watched Oh! My Lady
You could try Kou Kou Kyoshi it's J-Drama and not K-Drama though. Their are 2 versions of this drama a 2003 and a 1993 version both have an older man and a younger woman in a relationship. They both have a lot in common but the 93 version is better at least I think it is. I haven't really seen to many other series like these two drama's at least with an older male teacher with a female student and they actually have a relationship. It's not as light hearted as some of the drama's you mentioned so be prepared if you watch either of these. But the 2003 version will spoil certain things from 93 version because it has a few of same characters.
Coffee Prince Dating Agency Cyrano Dong Yi My Love Eun Dong Maids Cruel City Ooku (the series) Oh My Ghost Scholar Who Walks The Night The Greatest Love The Painter of the Wind
Older Female:
Witch's Romance What's Up Fox The Lover The Woman who still wants to get Married Crazy For You Last Cinderella I Need Romance 3 Dal Ja's Spring Kimi Wa Petto Secret Love Affair I do, I do
I don't have any suggestions for older male/younger female, however, The Noona Romance is my favorite trope. I've collected a bunch of titles at my blog here, https://thedramanoona.wordpress.com/the-noona-meter/
I saw someone recommended City Hall as an older female / younger male drama. It is not! The male lead is older than the female lead in both the drama and real life. That was such an odd suggestion on this list.
