Or maybe Barbara and Oswald planned for this and had him wearing some kind of kevlar prosthetic on his chest, complete with bloody squib packs.
There are any number of possible (if not plausible) justifications for what viewers saw.
We know Ed’s mind works more on the psychological side of evil-doing, evidenced extremely well through him making Oswald think his father’s ghost was haunting him, so it’s not out of the question that Ed used some kind of a trick bullet to make it look like Oswald was bleeding out.
Also, watching Oswald take a bullet to the gut absolutely had some shock value going for it, and it made sense for those characters’ combined arc as a whole, but it wasn’t such a convincing death that it positively cannot be explained in some other way when the show returns.