Old English þæron; see there + on. Similar formation in German daran.
Entries linking to thereon
there adv., conj.
Old English þær "in or at that place, so far as, provided that, in that respect," from Proto-Germanic *thær (source also of Old Saxon thar, Old Frisian ther, Middle Low German dar, Middle Dutch daer, Dutch daar, Old High German dar, German da, Gothic þar, Old Norse þar), from PIE *tar- "there" (source also of Sanskrit tar-hi "then"), from root *to- (see the) + adverbial suffix -r.
Interjectional use is recorded from 1530s, used variously to emphasize certainty, encouragement, or consolation. To have been there "had previous experience of some activity" is recorded from 1877.
on prep., adv.
"in a position above and in contact with; in such a position as to be supported by;" also noting the goal to which some action is or has been directed; "about, concerning, regarding; in a position to cover;" as an adverb, "in or into a position in contact with and supported by the top or upper part of something; in or into place; in place for use or action; into movement or action; in operation," Old English on, unstressed variant of an "in, on, into," from Proto-Germanic *ana "on" (source also of Dutch aan, German an, Gothic ana "on, upon"), from PIE root *an- (1) "on" (source also of Avestan ana "on," Greek ana "on, upon," Latin an-, Old Church Slavonic na, Lithuanian nuo "down from").
Also used in Old English in many places where we now would use in. From 16c.-18c. (and still in northern England dialect) often reduced to o'. Phrase on to "aware" is from 1877.