This code packet will help you to authenticate to O365 using Client Side Object Model(CSOM). It uses the  SharePointOnlineCredentials class to get clientContext of SharePoint. Let try it yourself.

// Namespaces
using System;
using System.Security;
using Microsoft.SharePoint.Client;

// String Variable to store the siteURL
string webUrl = "http://target.sharepoint.com";
// String Variable to store the user name
string userName = "name@tenant.onmicrosoft.com";
// String Variable to store the password
string password = "xxxxxxxxxx";

SecureString securePassword = new SecureString();
foreach (char ch in password)
  securePassword.AppendChar(ch);


using (ClientContextcontext = new ClientContext(webUrl))
{
  context.Credentials = new SharePointOnlineCredentials(userName,password);
  // To retrieve the web's title.
  context.Load(context.Web, w => w.Title);
  // Execute the query to the server.
  context.ExecuteQuery();
}