Gong HyoJin's amazing acting takes on a different, and slightly less 'intelligent' role in The Master's Sun - but pulls it off as flawlessly as she did her eccentric and complex character in It's OK, That's Love.
Both dramas have a lead character (Joong Won in The Master's Sun, Hae Soo in It's OK, That's Love) that sees themselves as a little bit superior and doesn't want to have much to do with the second lead initially, but turns into a comedic and cheeky relationship.
Both have psychological elements to them which can be very sad. Without giving too much away, a departing of a certain soul in TMS reminded me of a soul the metaphorically leaves in IOTL). They also deal with healing wounds and fixing problems of the past.
Both are funny and feel-good dramas.
TMS is a little more slow-paced but has a more complex plot; IOTL has more side-plots, a less complex main story but is more high-intensity and high-emotion.

These are both movies that can be easily recommended without giving another movie as an example, but both of these movies have similar themes. The biggest of which is probably injustice - the 'baddie' in the end, wins. The next biggest is family. The strong Father-Daughter relationship in both of these movies is touching and heart-wrenching at the same time. They both also a feel-good, but very sad movies. If you like one, I have no doubt the other would be to your taste.