Both are RomComs about unlikely couples. Both deal with mature relationship subjects. Both star Chun Jung Myung being extremely charming. There is almost a 10 year difference between the 2 dramas (What's Up Fox is definitely dated), but If you're looking to watch more Chun Jung Myung chemistry, he is equally unexpectedly crushable in both.
Oh My Ghost and Heart to Heart do not have similar plots, but they are both enjoyable to watch. The writing and directing along with the acting and chemistry worked well together to produce two wonderfully entertaining dramas. Although the plots are different, the female leads display a lovable awkwardness that subtly influences the charmingly over-confident male leads. Both dramas are definitely on my rewatch list.