Odoo provides External API: 

https://www.odoo.com/documentation/11.0/webservices/odoo.html

But it uses XML-RPC and I think it is not easy to implement xml-rpc client on Arduino.

The reason I ask if you can send http requests from Arduino it is you can easily create an simple HTTP endpoint on Odoo, without XML-RPC at all. What you need is a custom Controller: https://www.odoo.com/documentation/11.0/reference/http.html

Something like this:

class ApiController(http.Controller):
    @http.route('/api/dost/', methods=['post'], auth='public', csrf=False)
    def create_order(self, **kw):
        # Play with stuff, E.g:
        #env = http.request.env
        #werkzeug_request = http.request.httprequest
        #werkzeug_request.get_data()
        #str_data = werkzeug_request.data
        # and other parameters, please read Odoo document
        return "ok"

A few things to notice:

auth='public' : because you will not want Odoo to ask Arduino about authentication (username, password)

csrf=False: to disable csrf protection. You may consider another method to secure your API endpoint.

Now you can send a http from Arduino to http://your.com:8069/api/dost