Here is my sample URL rewriting definition for opencart SEO links that I hope to be useful on other developers running in NGINX. Notice that I have /opencart/ as the path of my opencart installation.
HTH
location /opencart/ { try_files $uri @opencart; } location @opencart { rewrite ^/(.+)$ /opencart/index.php?_route_=$1 last; } location /opencart/admin/ { index index.php; } location ~* (\.(tpl|ini))$ { deny all; }