I have an idea for a badass evil character. He’s an Orc Oathbreaker who lost one arm and eye fighting for Groomsh, and is basically really bad news. I imagined him wielding a greatsword with one arm, but I know it’s a Heavy weapon and you have to wield it with 2 hands. I could have sworn I’d seen a feat where you can wield a Heavy weapon with one hand but I can’t find it now. I can compensate for the spell casting hand movements with the War Caster feat, but is there a way for him to wield a great weapon with one hand in the content currently available or do I have to resort to homebrew?

Here's a secret tip from me to you:

Just say you have one arm.

Of course, you don't actually have one arm. You don't actually have any arms because the orc is not real. But it is totally, absolutely, wonderfully okay to just describe that your character has a certain disability or whatever that they have overcome to the point that they are just as functional as a normal character. And coincidentally, since they are just as functional as a normal character they can use the exact same rules and stats as a normal character. Maybe he only wears one boot and makes somatic gestures with his toes. You get the freedom to just make up reasons why he can do things just as well as someone with two arms and you don't have to introduce a single house rule to do it.

There was a Third Edition feat that let you wield a weapon that was normally two handed in one hand.  It was extremely broken and has not been brought to 5E.

I'd speak with your DM to have a feat to use 2H weapons with one hand, as long as you are missing the arm. In your case is not broken because it has more roleplaying effects than mechanical consequences. In fact you will be in disadvantage with other characters since you are using a feat only for cosmetic/RP purposes.