OAuth.module 3.x example - a test I made to verify the D7 port of the module - should work well on D6 as well though

vptest.info

name = VoxPelli OAuth Test
core = 7.x

dependencies[] = oauth_common
dependencies[] = http_client

vptest.module

<?php

function vptest_menu() {
  $items['vptest/request'] = array(
    'page callback' => '_vptest_test_request',
    'access arguments' => array('access content'),
  );
  $items['vptest/access'] = array(
    'page callback' => '_vptest_test_access',
    'access arguments' => array('access content'),
  );
  return $items;
}

function _vptest_test_request() {
  $consumer = DrupalOAuthConsumer::load('bVAU7jXKSL7vgnyayuiLZiGXZfoevZ8R', FALSE);
  if (!$consumer) {
    $consumer = new DrupalOAuthConsumer('bVAU7jXKSL7vgnyayuiLZiGXZfoevZ8R', 'N7LiwonhxUFs7RrwxEnizc2Doc7afPRx');
    $consumer->write();
  }
  $sig_method = DrupalOAuthClient::signatureMethod();
  $client = new DrupalOAuthClient($consumer, NULL, $sig_method);

  $request_token = $client->getRequestToken('http://sandbox.local/oauth/request_token', array(
    'callback' => 'http://d7.local/vptest/access',
  ));

  $request_token->write();

  $_SESSION['vptest_request_key'] = $request_token->key;

  $auth_url = $client->getAuthorizationUrl('http://sandbox.local/oauth/authorize', array(
    'callback' => 'http://d7.local/vptest/access',
  ));

  drupal_goto($auth_url);
}

function _vptest_test_access() {
  $consumer = DrupalOAuthConsumer::load('bVAU7jXKSL7vgnyayuiLZiGXZfoevZ8R', FALSE);
  $request_token = DrupalOAuthToken::loadByKey($_GET['oauth_token'], $consumer, OAUTH_COMMON_TOKEN_TYPE_REQUEST);

  $client = new DrupalOAuthClient($consumer, $request_token);
  $verifier = isset($_GET['oauth_verifier']) ? $_GET['oauth_verifier'] : NULL;
  $access_token = $client->getAccessToken('http://sandbox.local/oauth/access_token', array('verifier' => $verifier));
  $request_token->delete();

  $sig_method = DrupalOAuthClient::signatureMethod();
  $auth = new HttpClientOAuth($consumer, $access_token, $sig_method, TRUE, TRUE);
  $formatter = new HttpClientBaseFormatter(HttpClientBaseFormatter::FORMAT_JSON);
  $client = new HttpClient($auth, $formatter);

  $result = $client->post('http://sandbox.local/oauthlogin/api/user/info');
  print '<pre>';
  var_dump($result);
  print '</pre>'; die;
}