PowerShell and CSOM to authenticate to Office 365 and get a ClientContext object..

TopOfScript_PSCSOM.ps1

# replace these details (also consider using Get-Credential to enter password securely as script runs).. 
$username = "[email protected]" 
$password = "SomePassword" 
$url = "https://SomeSite.sharepoint.com"

$securePassword = ConvertTo-SecureString $Password -AsPlainText -Force 

# the path here may need to change if you used e.g. C:\Lib.. 
Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.dll" 
Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Runtime.dll" 
# note that you might need some other references (depending on what your script does) for example:
Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Taxonomy.dll" 

# connect/authenticate to SharePoint Online and get ClientContext object.. 
$clientContext = New-Object Microsoft.SharePoint.Client.ClientContext($url) 
$credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($username, $securePassword) 
$clientContext.Credentials = $credentials 

if (!$clientContext.ServerObjectIsNull.Value) 
{ 
    Write-Host "Connected to SharePoint Online site: '$Url'" -ForegroundColor Green 
} 

The line if (!$clientContext.ServerObjectIsNull.Value) is useless because it will always return true.