I'm chiming in a bit late but would like to contribute and confer regarding this issue. I'm no expert but keen to find better/correct techniques for this. Total newb in Octane.
I used to have this issue in C4d natively but found checking and reversing the normals if required - as moose mentions - solves this even when spline modelling. I'm guessing reversing the normals would be the answer regardless of you modelling technique?? Spline modelling is how I model bottles as I am from a retouching background, so I like to use Illustrator or the pen tool in C4d.
Builtdown mentions the air gap. I went through this with C4d and the physical renderer and the common theme I got here was that that the liquid spline needed to intersect the glass spline (effectively a little bigger) See pic attached. Now I'm not saying this is right or wrong and maybe this technique is the way to go with Lightwave and maybe even Octane.
Also mentioned is IOR, surely the correct IOR should used for different types of glass as well as liquids and we shouldn't have to 'fool' Octane with incorrect IOR's.
I watched a tutorial from these guys, it's worth a watch and they even have some studio set ups - one with a wine glass and wine - available for download/analysis:
This has helped but I am still not getting the results that I was getting in C4d's native physical render and TBH I use Octane most of the time bar when I am doing bottles - I go back to the physical render.
One further issue that I have is camera angles. Nearly all of the bottle renders I do are for commercial use and as such have a 'hero' composition ie. camera straight in front with a long'ish lens to reduce lens distortion and give a more isometric look. This gives a reflection of the floor in the bottom of the bottle, it's quite unsightly. If I use the physical render I can put a compositing tag on the floor and just un check 'seen by reflection' and the job's a good 'un. As, yet I haven't found a way to do this in Octane.
Having not been with Octane for long I haven't had too much time to try an address these issues and appreciate the dialogue...