Assassin's Creed Odyssey asks you to collect a lot of stuff. Money, friends, and sailors are just the start of it: There's also wood, leather, iron, glass, and a few other essentials needed to upgrade your gear and your ship. Then there's Orichalcum, a special glowing ore that one, and only one, merchant accepts for payment. The good news is, this one merchant has a store full of seriously awesome stuff. His name is Oikos of the Olympians and he can be found near the docks of Pilgrim's Landing in Phokis (don't worry, the main quest takes you there pretty quickly).
Like Xur from Destiny 2, Oikos carries a rotating stock of rare and powerful armors and weapons that can be purchased using Orichalcum. He also has a loot box which contains a mystery item but also has a chance of netting you one of the premium cosmetic items from the in-game microtransaction store. Once you've finally beaten the entirety of the Odyssey campaign, finding Orichalcum and buying stuff from Oikos will be one of the few reasons to play each week.
How to earn Orichalcum
Daily contracts (10 Orichalcum) and Weekly contracts (40 Orichalcum)
To start earning Orichalcum, head to any message board. There's one of these in every town at the base of a statue of Hermes (the messenger of the gods), and there's also a message board on the main mast of your ship. Orichalcum quests are set on timers, and two quests refresh daily and one more complicated quest pops up weekly. Looking through the list of lost goats and blood feuds against local bandits, find the quest marked by a swirly blue symbol.
The quest itself will be pretty normal and appear to be randomly generated just for these daily events. My quest, To Save a Mockingbird, was a call for help from a writer. Ugh, writers. Nothing but trouble. I found the writer on an isolated cliffside. She wrote a poem mocking a local man for having a small penis, and now he's coming with friends to kill her.
Here's where things get weird: Orichalcum quests appear to be more prone to bugs than other quests. Maybe it's because they're put together just for these events, or maybe I had a string of bad luck. But after I killed the angry dudes, the writer pegged me as hostile and started to run away. I ended up chasing her all over the island before I got close enough to trigger a totally normal conversation: "Ah, thanks for taking care of those men. Here's your pay. I certainly haven't been running away from you for several minutes."
Each daily quest completed earns you 10 Orichalcum, and a completed weekly quests earns you 40 Orichalcum.
Finding Orichalcum deposits while exploring
If you've completed all your contracts for the day, there are also hundreds of individual Orichalcum deposits out in the world. But good luck finding any. In around 45 hours of play, I've only come across three individual chunks of Orichalcum, and they were just sitting on random mountainsides like any other iron ore deposit, only glowing blue.
Fortunately, there's a simple way to highlight them all on the map. The in-game microtransaction store sells a map for just under $10 USD that'll mark every Orichalcum deposit on your map. Paying for that is for suckers, though, so instead take a look at this guide by Hold to Reset. They bought the map and took pictures of each zone so you can just reference that instead of coughing up your hard-earned cash.
What to buy with Orichalcum
There's exactly one man in all of Greece that accepts Orichalcum for payment: Oikos of the Olympians. Oikos has a small tent near the docks in Kirrha, a town in Pilgrim's Landing, the central region of Phokis. Phokis is the first nation on the first landmass you come to after leaving Kephallonia, so it's likely you've already been there. On your map, Oikos is marked by the same blue squiggle symbol you saw on the quest listing.
Oikos only has a couple of things to offer, but like the quests his stock changes weekly. Each week he'll offer three epic (purple) items and one legendary (orange) item. Picking out an individual item costs between 40 and 100 Orichalcum, with the legendary ones being the most expensive. A mystery loot box—sorry, I mean Olympian Gift—is cheaper at only 20 Orichalcum, but it's random whether you get something you want or not. The one thing that makes these lootboxes desirable is that there is also a small chance to earn one of the cosmetic items sold only in the microtransaction store. So if you don't want to cough up extra cash to ride Pegasus into battle (and who could blame you), this is the only way to get those cosmetics for free.
If you do decide to save your ore for another day, you can keep track of your total on the map screen, just to the right of your current gold amounts. Again, look for the blue squiggle—but this time, it's carved on the front of a glowing rock.