Optical multiplexing1,2,3,4,5,6,7,8,9,10,11 by creating orthogonal data channels has offered an unparalleled approach for information encoding with substantially improved density and security. Despite the fact that the orbital angular momentum (OAM) of light involves physical orthogonal division, the lack of explicit OAM sensitivity at the nanoscale prevents this feature from realizing nanophotonic information encoding. Here we demonstrate the viability of nanoscale information multiplexing utilizing the OAM of light. This is achieved by discovering OAM-dependent polarization ellipses in non-paraxial focusing conditions and hence synthetic helical dichroism resulting from the distinct absorption of achiral nanoparticles to the different order of OAM beams. Leveraging this mechanism, the application of subwavelength-scale focused OAM beams to self-assemble plasmonic nanoaggregates further enables six-dimensional optical information multiplexing, in conjunction with wavelength, polarization and three spatial dimensions. Our results suggest the possibility of multiplexing OAM division as an unbounded degree of freedom for nanophotonic information encoding, security imprinting and beyond.