OWA calendar settings include the option to make online meetings the default. You can control whether online meetings are the default at an organization and mailbox level. Outlook desktop relies on system registry settings to know if online meetings should be created. An add-in loaded is loaded automatically to insert the neceessary data to make a meeting online if necessary.
Changes coming in May and June will allow organizations to make online meetings the norm when created by OWA or Outlook mobile clients. You can control the feature at the organization level and allow individual mailboxes to override the organization setting.