In order to access your instance using HTTPS through NGINX, you need to follow these steps:

1. As root, install the Let's Encrypt client, certbot:

# apt-get update
# apt-get install software-properties-common
# add-apt-repository universe
# add-apt-repository ppa:certbot/certbot
# apt-get update
# apt-get install certbot python-certbot-nginx 

2. As root, request a certificate from Let's Encrypt (don't forget to change the email and the address of the server):

# certbot certonly --standalone -n --agree-tos
 -m [email protected] -d odoo.example.com

3. As root, install nginx:

 # apt-get install nginx

4. Create a configuration file in /etc/nginx/sites-available/odoo-ssl and add an upstream reference as follows:

upstream odoo {
    server 127.0.0.1:8069;
}
upstream odoochat {
    server 127.0.0.1:8072;
}

5. In the same file, add the rewrite rule to redirect http into the https website:

server {
    listen 80;
    server_name odoo.example.com;
    rewrite ^(.*) https://$host$1 permanent;
}

6. In the same file, add the following nginx configuration to serve the Odoo instance over https:

server {
    listen 443;
    server_name odoo.example.com;
    proxy_read_timeout 720s;
    proxy_connect_timeout 720s;
    proxy_send_timeout 720s;

    # Place configuration of steps 7 to 12 here    
}

7. Add the SSL configuration to the server block:

    # SSL Configuration
    ssl on;
    ssl_certificate /etc/letsencrypt/live/odoo.example.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/odoo.example.com/privkey.pem;
    ssl_session_timeout 30m;
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
    ssl_ciphers "ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384";
    ssl_prefer_server_ciphers on;

8. Add the log file configuration to the server block:

    # Add log files
    access_log /var/log/nginx/odoo.access.log;
    error_log /var/log/nginx/odoo.error.log;

9. Enable gzip by adding the following configuration to the server block:

    # enable gzip
    gzip on;
    gzip_types text/css text/scss text/plain text/xml application/xml application/json application/javascript;

10. Add proxy headers configuration to the server block:

    # Add Headers
    proxy_set_header X-Forwarded-Host $host;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto $scheme;
    proxy_set_header X-Real-IP $remote_addr;

11. Add reverse proxy configuration to the server block:

    # Manage longpolling on 8072 port
    location /longpolling {
        proxy_pass http://odoochat;
    }

    # Redirect requests to odoo server on 8069
    location / {
        proxy_redirect off;
        proxy_pass http://odoo;
    }

12. Enable the static cache by adding the following configuration to the server block:

    # Enable static cache
    location ~* /web/static/ {
        proxy_cache_valid 200 60m;
        proxy_buffering on;
        expires 864000;
        proxy_pass http://odoo;
    }

13. As root, link the configuration file in /etc/nginx/sites-enabled/:

# ln -s /etc/nginx/sites-available/odoo-ssl
 /etc/nginx/sites-enabled/odoo-ssl

14. As root, remove /etc/nginx/sites-enabled/default:

# rm /etc/nginx/sites-enabled/default

15. As Odoo, edit the production configuration file of the instance to enable proxy_mode:

proxy_mode = True

16. As root, restart your odoo instance and nginx:

# service odoo restart
# service nginx restart

17. As root, create a cron /etc/cron.d/letsencrypt file to ensure that the certificate will get renewed with the following content:

11 5 * * * certbot renew