Here is an oral proficiency rubric my colleagues and I have used.  It is theory-based and has been used in a peer-reviewed publication, so it is citable.

Data Oral Proficiency Rubric

Here is an oral proficiency rubric my colleagues and I have used.  It is theory-based and has been used in a peer-reviewed publication, so it is citable.

Data Oral Proficiency Rubric

Hi Lászlo,

You emphasize reliability- the accuracy of your measurement tools and the possibility of another researcher getting the same results if the variables of the test remain the same. It's also important to consider validity- the extent to which the test really measures what it sets out to measure. 

Another thing you will need to do is to define which aspects of oral proficiency you wish to focus on: fluency, pragmatic competence, language range, accuracy etc. 

You might find the following book helpful: Underhill, N (1987) "Testing Spoken Language

A Handbook of Oral Testing Techniques" .