Try Soilwork, perhaps. Amorphis is great as well.
Maudlin of the well
Ved Buens Ende
Brave Murder Day by Katatonia
Not 1:1 similliar, but you may like idk
Ne Obliviscaris (Check Intra Venus)
Gunman by Orden Ogan (kinda different but whatever)
And also Mastodon maybe (Jaguar God)
Between the buried and me is a must if you don't know them
Gojira, Lamb of God.. not as heavy, but good.. There's also Meshuggah
I dont want to disappoint you, but you are looking for the holy grail people try to find for 20 years.
Only Opeth is like Opeth.
i dont like metal but black sabbath, iron maiden and early metallica are nice.
I don't think there's Opeth besides Opeth
Gojira is not really like Opeth, but a wonderful band nevertheless and also progressive, so i guess it's kinda similar :D
The first Ulver record (Bergtatt)
The last few Novembre records (Materia + The Blue + Ursa)
Katatonia's Dead End Kings + The Fall of Hearts - and Mikael loves Katatonia anyway so you might like the rest of their catalogue too. He actually does the vocals on Brave Murder Day 'cause Jonas (Katatonia vocalist) blew his voice out on the album before
If you're into black metal, the last two Gris albums could also do the trick for you (Il était une forêt + À l'âme enflammée, l'âme constellée)
soen (has opeth's former drumer)
porcupine tree & steven wilson solo stuff (not metal but i think you'll like it if you like opeth)
dream theater maybe
have you listened to fear of a blank planet by porcupine tree? especially anesthetize, masterpiece. you shoud give it a go if you havent heard yet.
I don't think any band sounds similar to Opeth, their music is so diverse that it's hard to define. Here are some cool Progressive/Death Metal bands: