A preposition and prefix in The Etymologies of the 1930s glossed “above” and derived from the root ᴹ√ORO “up; rise; high” (Ety/ORO).
Conceptual Development: The Gnomish Lexicon of the 1910s had adverb G. or “on, on top” which could be used as a preposition meaning “on, onto” (GL/63), clearly also derived from ᴱ√ORO. As a prefix, G. or- meant “on, onto, up, in addition to, etc.” (GL/62).
Neo-Sindarin: For purposes of Neo-Sindarin I would only use or as an adverb/preposition/prefix for “above [not necessarily touching]”, while for “on [top of]” I would use S. po.
A prefix for “on both sides, across” in The Etymologies of the 1930s based the root ᴹ√AT(AT) “again, back” having to do with the number “two” (Ety/AT(AT)).
Conceptual Development: The root ᴹ√YAT “join” had deleted preposition N. iath “across” (EtyAC/YAT). In Tolkien’s later writings, prefixal “across” was generally thar or athra- based on the root √THAR (PE17/14), while S. ath- was used for “easy” (PE17/148).