Similar storyline where the FL travels back in time through an accident from the modern period to the past and found herself entangled between guys who sought after her. However, in Oh! My Emperor, the FL knows the ML before this happened.

In comparison, Oh! My Emperor is lighter and has more fantasy and magical elements than Scarlet Heart Ryeo, which is heavier and a lot more serious and political.

Both dramas gave the same feel and similar storyline so far. The leads are their each other's first loves whose beautiful relationship got deeply affected by their shared traumatic past and being related to a murderer (FL's father in Missing You while ML's father in Come and Hug Me). They met again several years later where the said story continues. It is also worth mentioning that the younger actors' acting and the main couple's chemistry of these dramas are both undeniably strong and impressive!

It's too early to tell what direction Mistress is going for but based on what I've seen so far, it shares certain similarities and vibes with Avengers Social Club.

Both dramas revolve around these four interesting people, mostly middle-aged women (Mistress's ensemble is portrayed a bit younger in comparison, though), who are bind by this unique and complex relationship—friendships that are in one way or another linked to revenge, which is likely provoked by their respective husbands, lovers/love interests (or [maybe] ex-husband/lover, in the case of the Mistress), and/or [possibly] other people. This aspect of the drama is a lot clearer in AVG since its focus is more on life and friendship (so it has more comedic and feel-good elements), unlike the Mistress which [might] focus more on mystery and romance (so it has more suspenseful and darker elements).

Moreover, both dramas are consist of only 12 episodes, so they are definitely binge-worthy!

At first, they may seem different: Love O2O, being focused on gaming and college life while It's Okay, That's Love, being more focused on psychology or healing.

However, after watching both of these great dramas, I'd say they somehow share a similar vibe, which may be because they both have:
-natural yet off the charts chemistry between the leads
-strong and mature relationship, where both sides support, respect, and trust the other person through thick or thin
-skinship and kissing scenes (or fluffy moments, in general) enough to make you root for the main couple (although IOTL is a lot better and intense on this one)
-admirable character and/or couple development (the lead/s definitely exhibit growth throughout the episodes which further helped strengthen their relationship)
-happy and satisfying ending

*there are probably more similarities, but these are all I could think of right now.

Both are introduced as melodrama and makjang which revolve around "identity swap" as the overarching theme. In Mask, it involves two strangers who look exactly alike (doppelganger) while in Nice Witch, it involves an identical twins.

Due to certain circumstances, both main female leads (who also happen to be the ones living a hard life and currently in a dire situation), were asked to take the other's identity and later became entangled with powerful people and series of misunderstandings.

Compare to Mask, Nice Witch has more comedic values in it.