The essential oils (OEs) of the leaves, stems, and spikes of P. marginatum were obtained by hydrodistillation, steam distillation, and simultaneous extraction. The chemical constituents were identified and quantified by GC/MS and GC-FID. The preliminary biological activity was determined by assessing the toxicity of the samples to Artemia salina Leach larvae and calculating the mortality rate and lethal concentration (LC50). The antioxidant activity of the EOs was determined by the DPPH radical scavenging method. Molecular modeling was performed using molecular docking and molecular dynamics, with acetylcholinesterase being the molecular target. The OES yields ranged from 1.49% to 1.83%. The EOs and aromatic constituents of P. marginatum are characterized by the high contents of (E)-isoosmorhizole (19.4–32.9%), 2-methoxy-4,5-methylenedioxypropiophenone (9.0–19.9%), isoosmorhizole (1.6–24.5%), and 2-methoxy-4,5-methylenedioxypropiophenone isomer (1.6–14.3%). The antioxidant potential was significant in the OE of the leaves and stems of P. marginatum extracted by SD in November (84.9 ± 4.0 mg TE·mL−1) and the OEs of the leaves extracted by HD in March (126.8 ± 12.3 mg TE·mL−1). Regarding the preliminary toxicity, the OEs of Pm-SD-L-St-Nov and Pm-HD-L-St-Nov had mortality higher than 80% in concentrations of 25 µg·mL−1. This in silico study on essential oils elucidated the potential mechanism of interaction of the main compounds, which may serve as a basis for advances in this line of research.