Hello there! 
The Oxford online placement test you mentioned is fairly easy and personally I wouldn't say it is a valid certification. I took it because some students were wondering how it would work and if it would be more useful than the IELTS, Cambridge English, etc. My answer is ABSOLUTELY NOT, they're not to be compared at all! 

It's far too easy and broad. Obviously, no speaking or writing is tested either. I guess it is a good rough guide to test a student's English but I wouldn't rely on it too much. 

I agree with other comments here.  

If you just want to demonstrate your English reading and listening levels for the purposes of your Italki profile, then it is ideal.

If you want to test your level, then there are free online resources out there that will suffice.

Some people complain that the accents of some of the people from England in the listening test are hard. If you know that you have a real problem with UK accents, then your listening score will probably come out lower than your true level.

You can take this test online through italki and it is not very expensive.  After you take the test you can add a badge to your profile and a small badge will be part of your discussion signature line.  Fun!

I think the test gives good results, from what I have seen here.