The "O" in Toluidine blue O refers to the fact that it is an "ortho" isomer of toluidine blue. An ortho isomer is a type of organic compound in which two substituents occupy adjacent positions on an aromatic ring. In the case of Toluidine blue O, the ortho isomer refers to the positioning of the two methyl groups on the toluidine molecule, which are located at adjacent positions on the benzene ring. The "O" is often included in the name of the dye to distinguish it from other Toluidine blue isomers, such as Toluidine blue S or Toluidine blue 3R.