The only thing these have in common is they are heavy beautiful scents 2 of my favourite but don't smell anything like each other Egoiste is one of the best Sandalwood fragrance that I have ever smelled opium is just a gorgeous spicy oriental
Both rich and kind of heavy, but Opium has a lot of spice blackcurrant and vanilla while Egoiste is mainly sandalwood cinnamon and rose, and I don't see them as particularly similar though I do think they are more or less in the same family.
For me Egoiste is the better scent.
They smell similar but in style, not in scent itself. Frags that share such style are Gucci Envy for men, CK Obsession for Men, YSL Opium, Boss Bottled, Chanel Allure for men, Chanel Egoiste etc. They all share the same vibe in a sense of style, but non of them actually smell alike.