Why is it O-Cl-O and not O-O-Cl?
Is there a relatively simple rule to solve (and understand) such
  relatively simple molecules?


The most electropositive atom is usually at the center of a molecule because


a "central atom" implies that there are multiple (more than 1) bonds to the central atom. If the central atom is relatively electropositive, then it will be better able to share its electrons and form bonds with other atoms, at least more so than an electronegative central atom would. 
electronegative atoms tend to carry multiple lone pairs of electrons. If this electronegative atom and all of its lone pairs were at the center of the molecular structure, then we would have many more destabilizing (lone-pair - bonding pair) electron-electron repulsions, then if all of these lone pairs were on the periphery of the molecule.