Recently I have checked an option to globally deploy custom Office templates in Office 365. Unfortunatelly there is no such an option but there are two manuall steps:
- Using SharePoint Online you can deploy templates in a SharePoint list and every time when you create a document inside of the list you can chose one of the templates.
- Upload the template in OneDrive for Business – each user needs to manually upload the templates
Additional information in the following two articles:
https://support.office.com/en-us/article/Set-a-file-template-for-a-document-or-form-library-8e0177eb-46bd-4346-9998-330c32733c25 – Set a file template for a document or form library
https://support.office.com/en-us/article/Publish-a-content-type-from-a-content-publishing-hub-58081155-118d-4e7a-9cc5-d43b5dbb7d02 – Publish a content type from a content publishing hub