I asked this same question on the JSON schema google group, and it was answered quickly. User fge asked that I post his response here:

Hello,

The current specification is draft v4, not draft v3. More specifically, the validation specification is here:

https://datatracker.ietf.org/doc/html/draft-fge-json-schema-validation-00

The web site is not up to date, I don't know why... I'll submit a pull request.

With draft v4 you can use this:

{
    "type": "array",
    "items": {
        "oneOf": [
            {"first": [ "schema", "here" ] }, 
            {"other": [ "schema": "here" ] }
        ]
    }  
}

For instance, this is a schema for an array where items can be either strings or integers (it can be written in a more simple way though):

{
    "type": "array",
    "items": {
        "oneOf": [
            {"type": "string"},
            {"type": "integer"}
        ]
    }
}

This is the correct answer. My corrected schema now includes:

"transactions" : {
    "type" : "array",
    "items" : {
        "oneOf" : [
            {
                "type" : "object",
                "properties" : {
                    "type" : {
                        "type" : "string",
                        "enum" : ["BUILD", "REASSIGN"]
                    }
                }
            },
            {
               "type" : "object",
               "properties" : {
                 "type" : {
                   "type" : "string",
                   "enum" : ["BREAK"]
                  }
               }
            }
        ]
    }
}

There are some errors in your schema. First, you are using properties for an array object. properties is a clause for objects, not arrays, so it will be ignored.

From json-schema v4, required is an array.

The following schema will require remindAt and comment properties for all items in the array:

{
    "type": "array",
    "items": {
        "additionalProperties": false,
        "properties": {
            "remindAt": {
                "type": "string",
                "format": "date-time"
            },
            "comment": {
                "type": "string"
            }
        },
        "required": ["remindAt", "comment"]
    }
}

You have defined your schema correctly, except that it doesn't match the data you say you are validating. If you change the property names to match the schema, you still have one issue. If you want to allow "toll" and "message" to be null, you can do the following.

{
  "type": "array",
  "items": {
    "type": "object",
    "properties": {
      "loc": {
        "type": "string"
      },
      "toll": {
        "type": ["string", "null"]
      },
      "message": {
        "type": ["string", "null"]
      }
    },
    "required": [
      "loc"
    ]
  }
}

However, that isn't related to the error message you are getting. That message means that data you are validating is not an array. The example data you posted should not result in this error. Are you running the validator on some data other than what is posted in the question?