It can be done with SharePoint Object Model 2013, the functions you need are inside the assembly: Microsoft.Online.SharePoint.Client.Tenant.dll, which is located at C:\Program Files\SharePoint Client Components\Assemblies after you install the SharePoint Client Object model 2013.
There's not much doc on this, but SharePoint Online Management Shell has the command to create the site collection, so I think it can be done with C# and figured it out. The code snippet shows how to do it.
using System;
using Microsoft.Online.SharePoint.TenantAdministration;
using Microsoft.SharePoint.Client;
using System.Security;
namespace SharePoint123
{
class Program
{
static void Main(string[] args)
{
//please change the value of user name, password, and admin portal URL
string username = "xxxx@xxxx.onmicrosoft.com";
String pwd = "xxxx";
ClientContext context = new ClientContext("https://xxxx-admin.sharepoint.com");
SecureString password = new SecureString();
foreach (char c in pwd.ToCharArray())
{
password.AppendChar(c);
}
context.Credentials = new SharePointOnlineCredentials(username, password);
Tenant t = new Tenant(context);
context.ExecuteQuery();//login into SharePoint online
//code to create a new site collection
var newsite = new SiteCreationProperties()
{
Url = "https://xxxxx.sharepoint.com/sites/createdbyProgram1",
Owner = "xxxxx@xxxxx.onmicrosoft.com",
Template = "STS#0", //using the team site template, check the MSDN if you want to use other template
StorageMaximumLevel = 100,
UserCodeMaximumLevel = 100,
UserCodeWarningLevel = 100,
StorageWarningLevel = 300,
Title = "CreatedbyPrgram",
CompatibilityLevel = 15, //15 means Shapoint online 2013, 14 means Sharepoint online 2010
};
t.CreateSite(newsite);
context.ExecuteQuery();
//end
}
}
}