For reverse proxy in front of Odoo, you need Nginx OR Apache so if you have Nginx you shouldn't need Apache.

BUT If you use Plesk, Nginx is controlled by Plesk which uses Nginx AND Apache together in order to separate requests for static files from requests for dynamic files like php scripts. This is from Plesk 11 release notes:

[...]nginx becomes a frontend web server that processes all incoming requests from site visitors. The requests are sent to Apache which, in turn, distinguishes requests for static and dynamic content.[...]

So, Plesk wants to control Nginx in a way that is not what you need in order to use it in front of Odoo. Although you may find a way to run Plesk with Odoo on the same server I wouldn't consider this good practice. I would keep theme separated. You can manage your domain's dns, mail etc on a server with Plesk and keep Odoo on another server.