Migrate commerce_order__order_items from CSV files

I'm writing a custom migration module from a legacy system. I have all of my orders and order items being migrated, but the commerce_order__order_items table is not being populated.

My orders migration:

id: my_order_data
label: My orders
migration_group: mygroup
migration_tags:
  - kentico
  - orders
source:
  plugin: csv
  ids: [OrderID]
  path: 'public://import/program/orders.data.csv'
  header_row_count: 1
process:
  order_number: OrderInvoiceNumber
  type:
    plugin: static_map
    source: type
    default_value: default
    map:
      commerce_order: default
  uid:
    -
      plugin: migration_lookup
      migration: my_user_data
      source: OrderCreatedByUserID
      no_stub: true
    -
      plugin: default_value
      default_value: 0
      strict: true
    -
      plugin: entity_exists
      entity_type: user
    -
      plugin: default_value
      default_value: 0
  state:
    plugin: default_value
    default_value: completed
  created:
    plugin: callback
    source: OrderDate
    callable: strtotime
  changed:
    plugin: callback
    source: OrderLastModified
    callable: strtotime
  placed:
    plugin: callback
    source: OrderDate
    callable: strtotime
  completed:
    plugin: callback
    source: OrderDate
    callable: strtotime
  mail: CustomerEmail
  store_id:
    plugin: default_value
    default_value: 1
  billing_profile_data:
    -
      plugin: migration_lookup
      migration: my_profile_data
      source: OrderBillingAddressID
      no_stub: true
  billing_profile/target_id:
    -
      plugin: skip_on_empty
      method: process
      source: '@billing_profile_data'
    -
      plugin: extract
      index: [0]
  billing_profile/target_revision_id:
    -
      plugin: skip_on_empty
      method: process
      source: '@billing_profile_data'
    -
      plugin: extract
      index: [0]
  total_price/number: OrderTotalNumber
  total_price/currency_code:
    plugin: default_value
    default_value: 'USD'
  total_paid/number: OrderTotalNumber
  total_paid/currency_code:
    plugin: default_value
    default_value: 'USD'
  adjustments:
    plugin: my_order_adjustment_shipping
destination:
  plugin: entity:commerce_order
  destination_module: commerce_order
migration_dependencies:
  required:
    - my_user_data
    - my_profile_data
dependencies:
  enforced:
    module:
      - my_migration

My order items:

id: my_order_item_data
label: My order items
migration_group: mygroup
migration_tags:
  - kentico
  - orders
  - items
source:
  plugin: csv
  ids: [OrderItemID]
  path: 'public://import/program/order-items.data.csv'
  header_row_count: 1
process:
  type:
    plugin: default_value
    default_value: 'default'
  order_id:
    -
      plugin: migration_lookup
      migration: my_order_data
      source: OrderItemOrderID
      no_stub: true
    -
      plugin: default_value
      default_value: 0
      strict: true
    -
      plugin: entity_exists
      entity_type: commerce_order
    -
      plugin: default_value
      default_value: 0
  purchased_entity:
    bar:
      plugin: static_map
      source: OrderItemSKUID
      map:
        377: 67
        378: 66
        ...
      default_value: 0
  title: OrderItemSKUName
  quantity: OrderItemUnitCount
  unit_price/number: OrderItemUnitPrice
  unit_price/currency_code:
    plugin: default_value
    default_value: 'USD'
  total_price/number: OrderItemTotalPriceInMainCurrency
  total_price/currency_code:
    plugin: default_value
    default_value: 'USD'
  created:
    plugin: callback
    source: OrderItemLastModified
    callable: strtotime
  changed:
    plugin: callback
    source: OrderItemLastModified
    callable: strtotime
  uses_legacy_adjustments:
    plugin: default_value
    default_value: false
destination:
  plugin: entity:commerce_order_item
  destination_module: commerce_order
migration_dependencies:
  required:
    - my_order_data
dependencies:
  enforced:
    module:
      - my_migration

I've tried to look into custom process functions, but I'm afraid I don't know where to really start with that. The commerce migrate module isn't terribly helpful with it's CSV example.