I am building an encounter for 6 or 7 PCs (mixed levels 4/5) with an Oni: it may turn into combat. Oni can be very difficult, even deadly, encounters for PCs at this level. What has me puzzled is how HP loss and the change shape feature work for an Oni.

Oni begins as a Large Giant, average 110 HP. What happens during the battle when the Oni changes shape into something else? My confusion comes from use of "polymorph" to describe the change shape ability. In the spell of that name the HP of the polymorphed form can be reduced without reducing the HP of the original form.

Combat Example

The party had done 76 points of damage (leaving the Oni with 34) when the Oni changes shape into an Ettin.

• Case 1: Oni's HP remain at 34



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  Case 2: Oni's HP are now 85 as an Ettin

  
  
  

  I am guessing the former

  
  
  

  
  

  Other than its size, its statistics are the same in each form.

  
  

  
  
  

  but am not sure. There does not seem to be a limit on the number of times an Oni can change shape, and it has another feature

  
  
  

  
  

  Regeneration. The oni regains 10 hit points at the start of its turn if it has at least 1 hit point.

  
  

  
  

Possible Problem: Immortal Oni

If Case 2 is true, the Oni would seemingly never run out of HP as it changes shape at will when it gets low, and also regenerates.

Change Shape. The oni magically polymorphs into a Small or Medium humanoid, into a Large giant, or back into its true form. Other than its size, its statistics are the same in each form. The only equipment that is transformed is its glaive, which shrinks so that it can be wielded in humanoid form. If the oni dies, it reverts to its true form, (MM, Oni)

How does it work: Case 1, or Case 2?

Case 1, the Oni remains with 34HP.

As you quoted in Shapechange:

Other than its size, its statistics are the same in each form.

So all things in the Oni's stat block remain exactly the same when it transforms, apart from its size and appearance. This means he remains at 34HP, and regens 10HP per turn as usual.

Case 1

Consider the wording of the Change Shape ability...

Other than its size, its statistics are the same in each form.

The only thing that changes with an Oni is its appearance and size. If it adopted the new form's hit points, it would say so. Consider, as a contrast, a few effects that do cause you to adopt a new form's hit points

Druid's Wild Shape:

When you transform, you assume the beast’s hit points and Hit Dice. When you revert to your normal form, you return to the number of hit points you had before you transformed.

All three types of Polymorph and Animal Shapes:

The target assumes the hit points of its new form.

Shapechange:

You assume the hit points and Hit Dice of the new form.

There is a long precedent of the D&D developers, particularly Jeremy Crawford (the voice of Official Rulings from Wizards of the Coast) making statements that clearly imply: "Things do what they say they do. If it did [X], then it would say so." See this answer for more details (though focused particularly on spells).

There seems to be some confusion with your reading of this ability

The Oni's change shape has nothing to do with the polymorph spell. In this description, polymorph is being used as a verb. The Oni is also not assuming any of the stats or abilities of other creatures.

Note the sentence:

Other than its size, its statistics are the same in each form.

This means that this transformation does not change the Oni's HP and does not grant it any abilities, the only thing that changes is the Oni's size. The choice of form that the Oni changes into does not really matter other than for visual purposes. This means that turning into an Ettin only really makes the Oni look like an Ettin, it does not grant it the Ettin's actions or other features.