Oath is a Runeword "ShaelPulMalLum" in Diablo 2 Resurrected (D2R). It requires these 4 runes to be inserted into a socketed item in the exact order: Shael + Pul + Mal + Lum . The eligible bases for Oath are the following: Swords, Axes, Maces . This is the only Runeword in the game that has X% Chance to cast Level X Bone Spirit on striking attribute.
OathOathBalrog Blade
Shael · Pul · Mal · Lum
One-Hand Damage: 46 - 66 to 232 - 330
Two-Hand Damage: 170 - 242 to 365 - 519
Required Dexterity: 87
Required Strength: 185
Required Level: 53
Increased Melee Range: 2
Base Attack Speed: 0
30% Chance to cast Level 20 Bone Spirit on striking
+50% Increased Attack Speed
+(210 - 340)% Enhanced Damage
+75% Damage to Demons
+100 to Attack Rating Against Demons
Prevent Monster Heal
+10 to Energy
+(10 - 15) Magic Absorb
Level 16 Heart of Wolverine (20/20 Charges)
Level 17 Iron Golem (14/14 Charges)
Socketed (4)
Trade items on d2trader.net

How to obtain Oath

The fastest way to get this item on Battle.net is via trading, you can use the Item Price Checker to see how much Oath is worth.
Another way of getting this Runeword is by farming the runes and the base. Find the drop rates of these items in the Item Database.

Oath bases

The best base for Oath is Balrog Blade. Below is a list of all eligible 4 socket bases for Oath.