Olea africana is used by some indigenous communities in Kenya to control gastrointestinal worms in animals. Plant-based anthelmintics are gaining popularity globally in the control of gastrointestinal worms in animals. The egg hatch inhibition assay was used to assess the in vitro anthelmintic efficacy of aqueous and ethanol leaf extracts of O. africana against the eggs of mixed gastrointestinal helminths in dogs. Probit regression was used to calculate the concentration of extracts that inhibited egg hatching by 50% (IC50). Albendazole was used as a control. Standard techniques were used to quantify the phytochemicals in the extracts. The aqueous extract had an IC50 of 1.85 mg/mL (1.64–2.10), and the ethanol extract had an IC50 of 0.25 mg/mL (0.23–0.26). Quantitative phytochemical analysis revealed that aqueous and ethanol extracts of O. africana contained alkaloids (19.40 and 61.60%), saponins (24.00 and 6.00%), phenols (0.95 and 1.28 mg/g gallic acid equivalents (GAE)), flavonoids (8.71 and 12.26 mg/g catechin equivalents (CE)), and tannins (67.30 and 76.30 mg/g of tannic acid equivalent (TAE)), respectively. O. africana has dose-dependent anthelmintic effects against mixed gastrointestinal worms in dogs. These findings support the traditional use of Olea africana as a treatment option for gastrointestinal worms in dogs.