Some ORM using PDO?

Definitely go with Doctrine.

If you're using PHP 5.3 there's a PHP implementation of ActiveRecord available now (link). It's up on github as well incase you want to watch or fork it. I checked the source on github and it does use PDO at its core to execute the queries.

I haven't used Doctrine or Propel. Propel looks straightforward and very nice (might have to give it a try) .. Doctrine seems to have a lot more to it than just ActiveRecord ORM. I'm a big fan of phpactiverecord (http://www.phpactiverecord.org/). It's simple, makes sense, and built for MVC architecture. I had used CodeIgniter's ActiveRecord in the past, this is similar: all php, very easy to setup a new connection, I particularly like the Dynamic finders, basically lets you put the field of the "WHERE" in the function instead of having to give an associative array. Propel might have this too, ex:

$book = Book::find_by_title('War and Peace'); 

versus:

$book = Book:find('all', array('conditions' => "title = 'War and Peace'")); 

Another option is phpDataMapper, which is a lightweight implementation of the DataMapper pattern.

• Doctrine (optionally with Symfony) - http://symfony.com/
• Propel (optionally with Symfony) - - http://propelorm.org/
• Zend_db (optionally with zend) - http://zend.com/
• CakePHP Using commonly known design patterns like MVC and ORM - - http://cakephp.org/

And for more ORM http://en.wikipedia.org/wiki/List_of_object-relational_mapping_software

I think one of best and simple ORM that uses PDO for php is
Redbeanphp :http://www.redbeanphp.com/