Further to Swindon Addick's post, the key difference is the no. of double cells subtended by RSPL. In O. brunneum it is typically 3 to 5 but can be up to 7, whereas in O. coerulescens it is normally none. Unfortunately, this feature cannot be determined from the photo. The next feature to look for is the size and colour of the pterostigma, in O. brunneum it is small and brown and in O. coerulescens it is large and yellow which matches the photo. There are a couple of other differences, one of the main being the colour of the frons which is whitish with a blue tinge in O. brunneum and brown in O. coerulescens. Again, though not discernible from the photo!
Hope this helps.
Shane