Another easy method of obtaining An Omnium Lost Ark Gold Star is to purchase one from the Spearfish hunting Guild Vessel located outside of Punika. If you are sailing from Punika's Punika docks, there is a chance that a merchant ship dubbed the Spearfish Hunting Guild Vessel will arrive to sell an Omnium Star. The star is available for sale to buyers for around 8000 Sun Coins, which you can get from Sailing Voyages or High Seas Coin Chests. If you're low on these chests, take on island quests, or the the Procyon's Compass adventure islands to gain more.
If you're a good source of Rapport items that you can give NPCs, achieving the Trusted Rapport status with Nia would be a great method to earn An Omnium Star. However, as you approach Friendly level with her you'll require a high level of virtue for advancement to Trusted rank. A way to avoid this is to reach Friendly stage one along with Nia by gifting her legendary gifts so that you can skip Friendly level two and skipping to Trusted level. If, for instance, you are on Friendly first level with Nia and have 7000 points, you can gift her 13 legendary gifts for her to skip Trusted. Unfortunately, building rapport with Nia is much more difficult by using this method, if you do not have the virtue stats required to go to Trusted.
Additionally, you can earn an Omnium Star Lost Ark Gold for sale from completing the Purification Ceremony every day, which is known as the Una task. While the actual task is not too challenging to complete, it will take you 21 real-world days to complete. You will need to start this as early as possible for the best chance of obtaining the Omnium Star swiftly. Thankfully, you can collect additional Omnium Stars in between these tasks.