Oxidation dyeing process using a substrate bearing at least one oxidation dye, several preservatives and an oxidizing aqueous composition The present invention relates to a process for the oxidation dyeing of keratin fibres, in particular human keratin fibres such as the hair, comprising a step of application, to said keratin fibres, i) of a substrate comprising on its surface one or more oxidation dyes, one or more thiolated reducing agents and one or more organic reducing agents of reductone type, and a step of application, to said keratin fibres, ii) of an oxidizing aqueous composition comprising one or more oxidizing agents. The present invention also relates to a process for preparing said substrate. The invention also relates to an element in sheet form, pretreated by a composition comprising one or more oxidation dyes, one or more thiolated reducing agents, and one or more organic reducing agents of reductone type.