I need to get a list of Sharepoint site collections using the Office 365 REST API. Is there a way to achieve this?
There are no REST endpoints at the farm level in SharePoint, so there is no REST endpoint for all site collections in your farm / lease.
Refer to: https://sharepoint.stackexchange.com/questions/113528/what-is-the-rest-endpoint-url-to-get-list-of-site-collections