Earthian

Both Earthian and Loveless stories are from the wonderful mangaka Kouga Yun. Both are unique and beautiful and portray an image of somewhat melancholic romantic relationships between boys. Other than that, the stories are completely different, but are both worth watching (and especially reading, since the anime shows only a portion of the whole story in both cases). 

Earthian and Angel's Feather are remarkably similar in almost all points. Both follow the same general plot of two opposing factions of angels fighting for supremacy. More, these two OVAs involve shounen-ai overtones (perhaps more pronounced in Earthian but very clear in AF as well) that make for some of the more dramatic moments. Earthian has a more dated look while AF is visually much more modern; seen side by side these two anime provide an interesting comparison.  

Shojo with sci-fi and supernatural elements about a group of peope from another planet that observe the Earth. 

The two protagonists of these '80s OVAs have a similar relationship with one another, and both shows feature a plot with an evil talent agency/record company. Also, the more "innocent" main character is voiced by the same person. 

Supernatural shojo with celestial beings living on Earth, with evil corporations and scientists as antagonist. Pretty violent by shojo standards. 

Similar theme and characters both anime involve people who have been sent to earth and have a "partners" that have more than a work related relationship. both have an obsessed antagonist  

These two animes aren't very simpler but they both have a main character with black wings. 

Earthian and Angel Sanctuary share the same general premise: angelic creatures set on earth engaged in a battle of cosmic proportions. More, both series have said supernatural entities acknowledging human emotions especially forbidden romantic relationships: in Earthian between two males and in Angel Sanctuary between siblings.  

They are both great shonen ai ovas.