Possible runeword not shown on item page

The database is correct - Oath can be made in Maces, but this does actually not include all Mace Class weapons.

Mace Class weapons in D2 are sub-divided into Clubs, Maces (proper Maces) and Hammers. Rune words typically specify exactly which sub-class of Maces they can be made of.

Clubs include Club, Spiked Club and their exceptional/elite variants
Maces included Mace, Morning Star, Flail and their exceptional/elite variants
Hammers include War Hammer, Maul, Great Maul and their exceptional/elite variants

You can see this sub-division at the old Arreat Summit website on the Maces page:
https://classic.battle.net/diablo2exp/i ... aces.shtml (normal)
https://classic.battle.net/diablo2exp/i ... aces.shtml (exceptional)
https://classic.battle.net/diablo2exp/i ... aces.shtml (elite)

The rune words page also has some clarification about this:
https://classic.battle.net/diablo2exp/i ... ords.shtml
Specifically, the section that detals the required item type for formulae;

Blizzard wrote:

...
Clubs = Club, Spiked Club, and Exceptional/Elite versions.
Hammers = War Hammer, Maul, Great Maul, and Exceptional/Elite versions. These will not work in mace recipes even though they are listed on the mace page.
Maces = Mace, Morning Star, Flail, and Exceptional/Elite versions. This does not include any other types of Maces. Make sure you read about Hammers above. They are not "maces" in the case of Runewords.
...

In this case, Oath can only be made in Maces, not Clubs nor Hammers. And the only Maces that allows 4 sockets is Flail/Knout/Scourge.
It's the same thing with Heart of the Oak - it can only be made in Maces too, meaning Flail/Knout/Scourge.
And the other way around for Beast - it can be made in Hammers (like War Hammer and Maul) but not in Maces (like Flail).
+ many more examples