If you need to guarantee a certain version of Octavian you can explicitly ask for it via add Octavian@0.21 and will get an error if there’s a conflict.
If you’re on 1.7, your problem is likely that you are on a very old Turing version, as Turing doesn’t work yet on 1.7 so falls back on an old version.
Also if you happen to install your packages in the default environment but aren’t actually using Turing and Octavian at the same time, you can get around the conflict by working in project environments (which you should do in any case)