Low phosphorus (P) availability is a critical impediment to agricultural use of P-fixing soils. De-oiled two-phase olive mill waste (DTOMW) may be used as a soil amendment that can increase P availability and consequently improve crop production. The aim of this study was to investigate the effect of DTOMW amendments to soil on the fractionation, sorption, and buffering capacity of P in soils collected from a typical olive grove amended in the field with 27 and 54 Mg ha−1 of DTOMW for 7 years. Cumulative and residual effects were evaluated in the last year of DTOMW application and 2 years subsequent to the last application. A modified Hedley fractionation scheme was used to evaluate the change in soil P forms, whereas the potential for sorption of P was evaluated by applying the surface Langmuir models to sorption isotherms. The P fractionation results indicated an increase in all P fractions (P < 0.05) in the studied samples. The order of the rates of increase in concentrations of P fractions was HCl-Pi > NaHCO3-Pi ≍ NaOH-Po > HCl-Po ≍ NaOH-Pi > H2O-Pi > NaHCO3-Po > residual P > H2O-Po in the cumulative year, whereas in the residual year, the order of increase was HCl-Pi > NaOH-Po > NaHCO3-Pi > H2O-Pi > NaOH-Pi > HCl-Po > residual P > NaHCO3-Po > H2O-Po. The DTOMW amendments significantly decreased the P sorption maxima (P < 0.05). Most of the total P sorption potential was attributed to low-energy sites, independent of the amendment applications received. The DTOMW soil application decreased indices of P-binding intensity at both high- and low-affinity sites, decreased the P equilibrium buffering capacity, and increased the equilibrium P concentration, indicating that P availability increased with increasing DTOMW rate and also with increasing humified organic matter fraction. This study has shown that in olive grove soils, successive DTOMW amendments may be an effective management practice for controlling their ability to fix inorganic P and increase P availability for the crop for at least 24 months after its application.