The RN Obs aptitude test is compiled from a slightly different selection elements of the overall battery than RAF WSO. I am led to believe therefore that scores a not directly transferable between the two. To get your Obs score you need to contact a RN AFCO and apply I believe when they will then ask Cranwell for your relevant RN Obs conversion.
Generally though, they are of a similar principle, i.e. workload, ability to multi task etc. So although it may convert to a pass, it won't be spectacular. I also believe, and I'm sure others will correct this if wrong, that you have to have passed ATC as well to be considered for AIB.