I defined the following schema:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "$id": "https://example.com/schemas/test",
  "type": "object",
  "properties": {
    "A": {
      "type": "string"
    }
  },
  "oneOf": [
    {
      "properties": {
        "B": {
          "type": "integer"
        },
        "C": {
          "type": "boolean"
        }
      }
    },
    {
      "properties": {
        "D": {
          "type": "string"
        }
      }
    }
  ]
}

The idea is that the schema contains A, and either B,C or D. None of the elements are required.

The following instance fails to validate:

{
  "A": "world"
}