Bansh33 Thanks for the reply but I have a pretty extensive set of virtual servers and websites behind my nginx and I won't tare it all down to install another reverse proxy.
Arsalan Shah I will look into this when time permits. Thanks for the info.

I haven't tried using nginx, but I use ngrok as a reverse proxy with ossn without a problem. I have a local IP for a virtual machine running ossn, and I then launch ngrok with the command ngrok http -hostname=www.mysite.com 192.168.x.x:80 and it works without a problem or without any added config to either ngrok or ossn. The only thing needed was to set the url to http://www.mysite.com in ossn.config.site.php. I'm not running a site with an ssl cert for ossn, but I have another site on ngrok which is using an ssl cert and works fine when accessing it via https on abrowser. I suspect there wouldn't be any issue using an https site with ossn over ngrok, you'd just need to change the url to use https in the config file. Unfortunately I haven't really used nginx as I said earlier, so I can't be much help with that, but if your only need is a reverse proxy, you may want to look into alternatives like ngrok or localtunnel.

Does your ngnix setting HTTP_X_FORWARDED_PROTO header for https? if so try to replace the ossn_redirect_absolute_url in ossn.lib.javascripts.php with below

function ossn_redirect_absolute_url(){
    $baseurl    = ossn_site_url();
    $parts      = parse_url($baseurl);
    $iswww      = preg_match('/www./i', $parts['host']);
    $host       = parse_url($_SERVER['HTTP_HOST']);
    $redirect   = false;
    $port       = "";
    if(!isset($host['host'])){
        $host = array();
        $host['host'] = $_SERVER['HTTP_HOST'];
    }
    if(isset($parts['port']) && !empty($parts['port'])){
        $port = ":{$parts['port']}";
        if ($parts['port'] == ':80' || $parts['port'] == ':443'){
            $port = '';
        }
        if($parts['port'] !== (int)$_SERVER['SERVER_PORT']){
            $redirect = true;
        }
    }
    if(isset($_SERVER['HTTP_CF_VISITOR']) && strpos($_SERVER['HTTP_CF_VISITOR'], 'https') !== false) {
        $_SERVER['HTTPS'] = 'on';
    }
    if (isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] === 'https') {
        $_SERVER['HTTPS'] = 'on';
        $parts['scheme'] = 'https';
    }
    if(empty($parts['port']) && isset($_SERVER['SERVER_PORT']) && !empty($_SERVER['SERVER_PORT'])
        && $_SERVER['SERVER_PORT'] !== '80' && $_SERVER['SERVER_PORT'] !=='443'){
        $redirect = true;
    }
    if($parts['scheme'] == 'https' && empty($_SERVER["HTTPS"])
        || (!empty($_SERVER["HTTPS"]) && $_SERVER["HTTPS"] == "on" && $parts['scheme'] == 'http')) {
        $redirect = true;
    }
    if(($host['host'] !== $parts['host']) || $redirect){
        header("HTTP/1.1 301 Moved Permanently");
        $url = "{$parts['scheme']}://{$parts['host']}{$port}{$_SERVER['REQUEST_URI']}";
        header("Location: {$url}");
    }
}