This research work is based on the isolation of cellulosic fibers from Tunisian olive sawdust (TOS) by alkaline treatment using a solution of NaOH at different concentrations: 5% w/v (TOS5) and 20% w/v (TOS20). The valorization of these cellulosic supports was assessed by the adsorption ability of methylene blue (MB), chosen as a model compound of organic dyes. SEM and FTIR analyses have shown that alkaline treatment produces a delignification of TOS and an elimination of hemicellulose. A transformation of cellulose I to cellulose II was shown by XRD following the decrease of the crystallinity indices passing from TOS5 (~37%) to TOS20 (~30%). The spectroscopic study confirmed that the intermolecular and intramolecular hydrogen bonds have a primordial role in the forms and the structural stability of the cellulosic substrates. Thereby, the deconvolution of OH stretching vibration band confirmed the transition from cellulose I to cellulose II by the occurrence of a new band at 3,080 cm-1 for TOS20. The increase of iodine adsorption values (ISV) from 40 mg·g-1 for TOS5 to 65mg·g-1 for TOS20 is behind the improvement of the accessibility of cellulosic support with alkaline treatment of TOS. The adsorption of MB onto the surface of cellulosic substrates in aqueous mixtures was studied quantitatively. The corresponding isotherms in basic medium have shown the formation of a monolayer following the Langmuir model characterized by 14mg·g-1, 52 mg·g-1 and 76 mg·g-1 onto the TOS, TOS5 and TOS20, respectively. The desorption efficiency of MB in acid medium increased with 37%, 57% and 71% for TOS, TOS5 and TOS20, respectively. Hence, the extracted cellulosic fibers are likely to be a promising adsorbent for the removal of organic dye from aqueous solutions.