The invention relates to a method for manufacturing a final optical fiber preform by overcladding, said method comprising the steps of providing a primary preform; positioning said primary preform within at least one tube, wherein the at least one tube partly covers the primary preform to create a zone to be overcladded, being an overclad zone, which overclad zone is located on the primary preform outside the at least one tube; injecting a gas into the annular space between the primary preform and the at least one tube under overpressure relative to the pressure outside the at least one tube; overcladding the primary preform in the overclad zone with an overcladding material using an overcladding device. The invention also relates to an apparatus for carrying out the method. The invention allows overcladding a primary preform at low cost while maximally limiting the incorporation of impurities into the silica overclad.