That is to say when you did you initial install, you were working on a fully updated device. If Oreo has worked fine for quite a time no Stock ROM should be required.
If you had good reason to believe that you want to refresh the foundation to the full Stock ROM then Odin is good. Heimdall is often found difficult if you have not fully seen what has to happen before you start.
In the Upgrade link provided by @claudiousse there is no mention of returning to Stock ROM.
EDIT I am sorry @deepee, I have to say I have no certainty about this because I would be surprised if Samsung were adding to the vendor parts of the firmware files so long after the device was designed and built. (In a quick check I could not find a reliable source for the date vendor patches stopped.) Updates however will have been released to apply the latest Android security patch.
An interesting cross-check on you current Oreo build would be to look in Settings > About phone > Android version > Vendor security patch level. I guess you still would not know for sure unless you did the Odin firmware flash followed by /e/ Q and see if the Vendor security patch level did update. Or unless you posted your current patch level here and someone with the device reported having an update on that.