On

spatial preposition

• In X is on Y there is minimal contact, if Y disappeared, X would fall.
  • The book is on the desk.
  • The ornaments are on the tree.

particle

pre-established circuit marker, with off as its antonym. transmission of energy (turn SO on / turn SO off)

turn on/off

get on/off the bus

on schedule, on time

on the clock, online, on the phone, on TV, on camera, on the air, etc.

turn SO on (Cf. Nora Jones), lead on, get on (a project)

Continuative particle

Move on, lead on, carry on, get on (in age)

evolution

introduced -ing nominals (historically)

On Hunting --> a-hunting --> hunting.

Combined with other prepositions

on (the condition) less than --> unless

on till --> until

on --> a (ahead, above, afore, etc.)