We study the solubility properties of regioregular oligo(3-alkylthiophene)s in tetrahydrofuran solvent as a function of their alkyl chains length by an atomistic investigation based on model potential molecular dynamics. We make use of the Flory-Huggins theory that is typically used to study the miscibility of macromolecules and that is here applied for the first time to study the solubility of conjugated conducting polymers in a typical organic solvent. The properties of the isolated solvent and polymer are correctly reproduced, and the calculated solubilities of the oligo(3-alkylthiophene)s in tetrahydrofuran as a function of their side chains lengths are in agreement with available experimental data. Present investigation shows that the atomistic approach based on molecular dynamics is a powerful tool to study the solubility of alkylthiophenes in molecular solvents.