In career mode, I have about half of the 90-science level R&D nodes unlocked (and everything of lower levels). When hovering over most tanks and looking at the info card, LH2O (Liquid Hydrogen + generic Oxidizer) is listed as an available tank configuration; however, it's not available in the actual tank switch options.

I have a lot of mods installed, but it's all through CKAN, so either this is a bug and it should be working, or there's an unlisted incompatibility. I looked through the metadata > relationships tab for every mod I could think might be affecting this, and didn't see any marked incompatibilities that seemed relevant.

I checked in Sandbox mod, and everything seems to be working properly there. I suppose it could be I haven't unlocked the right R&D node yet, but if that were the case I'd think LH2O wouldn't be listed in the parts-list info card yet and I wouldn't already have access to LH2O-burning engines (of which I have several unlocked).