Oxacillin is a penicillinase-resistant, narrow-spectrum beta-lactam crystalline antibiotic of the penicillin class, resistant to beta-lactamases. The main purpose of this paper was to develop and apply a new spectrophotometric method in the visible (VIS) field for the analysis of pure Oxacillin from various pharmaceutical samples. Oxacillin reacted completely with 1.10 Phenanthroline 0.2% alcoholic solution in the presence of an aqueous solution of ferric chloride (FeCl₃, 6%), which led to the quantitative formation of a intense bright yellow complex with a faint orange tint, after heating the solutions for 20 minutes at 70 ºC constant temperature. The intense bright yellow colored synthetized complex was then spectrophotometrically determined at λ = 420 nm corresponding to its absorption maximum, in relation to double-distilled water as a blank. One solid pharmaceutical capsule officially contained 500 mg pure Oxacillin, as reference value. As a result of the experiment, 491.308 mg of pure Oxacillin/solid capsule was obtained. This amount corresponded to a percentage content of 98.262 % determined by pure sodium Oxacillin and calculated per solid capsule of pharmaceutical product. The obtained result was very close to the reference value which was 500 mg of pure Oxacillin. The relative percentage deviation (relative percentage procedural error) was only E = 1.738 % to the official value and it fit perfectly within the official normal limits of values indicated by the Romanian Pharmacopoeia, 10th Edition, and by the European Pharmacopoeia Rules (± 5 %). Statistical analysis revealed a very good linearity of the visible spectrophotometric method for pure standard analyzed solutions in a large concentration range, from 1,20 μg/mL to 36,00 μg/mL. The linear regression coefficient R² was 0,999504 and the correlation coefficient R was 0,999752; as R² ≥ 0.9990 and R > 0.9990, they were found to be within the normal official range of values.