For grinding Orns, my suggestions (written for tier 6 & 7 but most apply for other tiers) are:
• Make sure to use both Lucky Coins everytime you play, you'll very likely get way more than you could ever use as long as you play to some extent.
• For items, you can get the Deathbringer (Thief weapon) and the Reapers Robe (Mage armor) from Ankou, it's a T7 boss, however you can find them in the world in Tier 6. Both items give a 1.05x (5%) boost to Orns regardless of quality. There's also the Bag of Treats, which is a T5 accessory dropped by The Fool and Pumpkinhead raids, it gives a 1.03x boost to Orns, along with a boost to Gold, EXP, and item drop rate. However it should be noted that items like these that boost things such as Orns, Gold, item drop rate, and EXP, don't effect Raid rewards.
• Quests are always a great way to earn Orns as well, particularly quests for Gauntlets and Mimics. I recommend accepting every quest from Inns and NPCs that you pass by, aside from the rare bad quests such as ones for handing in rare potions or gauntlet keys that only give a few hundred Orns.
• Speaking of Gauntlets, they are also really good for earning Orns (though that might not be a viable source of Orn grinding for you yet depending on your tier, how many gauntlet keys you have, and how frequently you get them), however in Tier 7 and up Gauntlets are something you can usually do quite frequently.
• Raids are definitely a great source for Orns, Gold, and EXP. Make sure to participate in all the raids you can damage, and always make sure to at least fight current event Raids. In general you wanna focus on raids you can do a decent amount of damage on in a moderately quick time, which would likely be raids at your tier or one below.
• Regular world enemy grinding can be good as well of course, especially if you can kill lots of daily quest monsters.
• Completing Achievements can also give a good amount of Orns as well, so it's worth trying to go for most of them, but don't worry if certain ones are more slow to do than others; particularly Master Blacksmith, Skilled Herbalist, The Explorer, Master Duke, and Champion. I highly recommend making it a point to complete all of The Polymath achievements though as you get back more Orns in the end than what you spent. Other than that, the ones I recommend going for the most are: Frequent Visitor, Monster Slayer, Boss Slayer, Quest Seeker, and Gaunlet Master.
Probably the best way to earn lots of Orns quickly though is by being a passenger in a car/bus and killing all the bosses you come across, along with taking out the Area Guardians in the Explore tab (which you should do even when not travelling in a vehicle).