Oro is the husband of Applebloom and the father of her kids. He is mentioned in Ghosting but doesn't appear in a story yet. When he does, it will be in the list below:

https://www.fimfiction.net/bookshelf/1294304/bright-future?order=latest

Who is Cuore d'Oro and where did he come from? A lot of ponies ask these questions, but only one actually knows the answer, and she isn't telling. What he is, as far as ponies know, is a very talented gardener, an unusual talent for a unicorn as that's typically an earth pony trait. His botanical magic leaves him very knowledgeable about plants and he can even make seeds mature in the blink of an eye, but the full extent of his abilities is generally unknown. He's also been known to be quite the archer and totally untrained in classical magic like other unicorns. Charmer, looker, mare's stallion extraordinaire, he is the envy and the desire of many but belongs to one, his beloved Applebloom. If there were rumors spread about Applejack's husband appearing out of nowhere and joining the family suddenly, they are even wilder and more misguided concerning Oro. Of the few who do know the solitary stallion, they say he's generally polite and easy to talk to, but never open about himself. Oro's first name ought to be 'Mistero,' with a heart as guarded as his.