You can refer to Farhady, Jafarpour, Birjandi book or Fulcher and Davidso. Different methods and criteria of testing oral proficiency are provided there.
this file can also help: https://www.teachers.cambridgeesol.org/ts/digitalAssets/118613_Assessing_Speaking_Presentation_Slides.pdf
I believe you can find very useful information in "How to teach speaking", a book written by Scott Thornbury. You can also find interesting information in Jeremy Harmer's "How to teach English" (2007); concretely, chapter 9. Finally, you could have a look at "NCLRC The essentials of language teaching". Here you can find principles, strategies, resources, etc. http://www.nclrc.org/essentials/speaking/spindex.htm
I suggest the format which is given in Penny Ur book ( I do not remember the name, however, you can find it in most of the lranian libraries. It rates based on two categories of accuracy and fluency.
Basically when we conduct any assessment or test for our students' oral proficiency is to know their ability and performance in learning and speaking the Target Language.
Their proficients are measured with a speaking rubric which will help them to reflect how much how they have learnt, motivate and decrease their anxiety level.
I believe it's very important to take the situation in which you are assessing your students' proficiency level into account. measuring their ability while doing some free, not controlled predefined situations seem to give a more exact picture.
I believe casual and friendly oral interaction that takes place in a love-full, fun-filled and above all fear-free zone will be of true help in testing oral proficiency. Formal and well-structured dialogue, formal interviewing, posing some questions and deciding the oral fluency conducted in a teacher-student authoritarian setting is often found to be defective in my experience..but still it is okay for the bright students ..but not for struggling and weak students...If no other option.., then choose the best..but please employ both formal and informal oral assessment devices. Thank you.