Yes, you can use app-only policy to access SharePoint Online. First step is to allow it in your app.manifest.

Second step may be little tricky. I did it about a year ago and it required to modify TokenHelper.cs file.

My case was simple, as I was making exclusively app-only calls. I did not use SharePointContext or SharePointContextFilterAttribute, so I removed them from my code. I relied on TokenHelper to create ClientContext. By default it was throwing exceptions when no user was logged in (NullReference if I recall correctly). Eventually using debugger I was able to find place where this exception originated and modify TokenHelper to ignore user.

I know this answer isn't too specific, but I just wanted to point out that it is possible to do it this way.

Yes, you can use app-only policy to access SharePoint Online. First step is to allow it in your app.manifest.

Second step may be little tricky. I did it about a year ago and it required to modify TokenHelper.cs file.

My case was simple, as I was making exclusively app-only calls. I did not use SharePointContext or SharePointContextFilterAttribute, so I removed them from my code. I relied on TokenHelper to create ClientContext.

I see now, that with current version of TokenHelper.cs no modifications are required. I've created new SharePoint App project in Visual Studio, removed files mentioned earlier and edited HomeController. user.Title returns "SharePoint App".

public ActionResult Index()
{
  var uri = new Uri("https://sample.sharepoint.com/sites/sample/");
  var accessToken = TokenHelper.GetAppOnlyAccessToken(TokenHelper.SharePointPrincipal, uri.Authority, TokenHelper.GetRealmFromTargetUrl(uri));
  using (var context = TokenHelper.GetClientContextWithAccessToken(uri.ToString(), accessToken.AccessToken))
  {
    var web = context.Web;
    var user = web.CurrentUser;
    
    context.Load(user, u => u.Title);
    context.ExecuteQuery();
    
    ViewBag.UserName = user.Title;
  }
  
  return View();
  }
}