In Office 365, you are provided with a “parent” site collection that you cannot delete/ remove. Any other site collection created is considered a site collection down level (sub site collection) to the parent site collection (for the purposes of being a shared space.) Typically (if required) you would link the breadcrumbs together by creating a portal site connection. This connection essentially adds the parent site collection to the breadcrumb navigation on the sub site collection. If you had an opportunity to get in to the early beta for SharePoint Online, you may have started creating sub site collections. I noticed that when I linked the sub site collections that the top level site collection did not show up in the breadcrumbs. In fact, the only way it would show was if you were in a site within the sub site collection. Here is an example:
Site within the sub site collection:
I’m not sure why you can see the parent site collection when in a site of the sub site collection. This issue may be related to an early deployment, as any new site collection I create no longer has this problem. So if you have an early beta of SharePoint Online and are experiencing this problem, I would suggest you try creating another site collection and add the parent site collection in the portal site connection area and then test it. From everything I am seeing, it appears to work in the latest update of SharePoint Online.