We have studied the effect of particle wettability on the structure of planar monolayers of micron size monodisperse silica particles at the octane-water interface. The particles' wettability was adjusted by silanization of their surfaces with dichlorodimethylsilane; thus their contact angle at the octane-water interface was in the range 70-150degrees. The modified particles were spread at the interface, and the monolayer structure was studied by direct microscope observation. A very distinct difference in the structure of monolayers depending on the particle contact angle has been observed. Very hydrophobic particles with large contact angles gave well-ordered monolayers at interparticle distances larger than 3 particle diameters. Such well-ordered monolayers in the case of silica particles are reported for the first time. The monolayers of less hydrophobic particles (with contact angles smaller than 115degrees) were totally disordered and aggregated. The disorder-order transition in the monolayer structure occurred in the narrow interval of contact angles between 115degrees and 129degrees. The pair interaction energies due to dipole-dipole or Coulombic repulsion through the oil have been calculated and compared. We have concluded that the disorder-order transition and the ordering of the monolayers at large interparticle distances cannot be explained by dipolar repulsion between particles, but they both are consistent with Coulombic repulsion acting through the octane phase as a result of the presence of charges at the particle-octane interface.