How could use the data of one action method in another action method

RedirectToAction returns a response to the client telling it to make a new request to the specified action.

The ViewBag contents are not persisted between requests. Anything you add to the ViewBag before returning a redirection response will not be available when the browser makes the new request.

There are a couple of options here:

1) Don't use RedirectToAction:

public ActionResult SenddOTP()
{
    ...
    ViewBag.otp = strrandom;
    return SendEmailtoUser();
}

private JsonResult SendEmailtoUser()
{
    string OTP = ViewBag.otp;
    bool result = SendEmail("user@gmail.com", "Hello", OTP);
    return Json(result, JsonRequestBehavior.AllowGet);
}

Or:

public ActionResult SenddOTP()
{
    ...
    return SendEmailtoUser(strrandom);
}

private JsonResult SendEmailtoUser(string OTP)
{
    bool result = SendEmail("user@gmail.com", "Hello", OTP);
    return Json(result, JsonRequestBehavior.AllowGet);
}

The browser will make a single HTTP request, and there will be no redirection involved.

2) Use the TempData storage:

public ActionResult SenddOTP()
{
    ...
    TempData["otp"] = strrandom;
    return RedirectToAction(nameof(SendEmailtoUser));
}

public JsonResult SendEmailtoUser()
{
    string OTP = TempData["otp"] as string;
    bool result = SendEmail("user@gmail.com", "Hello", OTP);
    return Json(result, JsonRequestBehavior.AllowGet);
}

This will still use two HTTP requests.

NB: In this case, you would need to validate that the TempData value existed, since the user could navigate directly to the SendEmailToUser action without going via the SenddOTP action.

NB2: There's only one "d" in "Send". :)