Monolayers of the organic donor OMTTF (octamethylene-tetrathiafulvalene) were grown on a Ag(100) substrate and on the iodine c(2x2) superstructure on Ag(100) by organic molecular beam deposition (OMBD) under ultra high vacuum (UHV) conditions. The films were characterized by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). In the monolayer regime the molecules grow on both substrates in an ordered structure. By comparing the degree of long-range order and the geometrical structure observed on Ag and on iodine with that of OMTTF-TCNQ we discuss the interplay between the different interactions and long-range order in epitaxial growth.