Target trial emulation is increasingly used for estimating treatment outcomes in observational cohorts.<h4>Objectives</h4>To describe hormone therapy and OFS treatment patterns (aim 1), examine the association between adding OFS to tamoxifen (TAM) or aromatase inhibitor (AI) and survival (aim 2), and examine the association between duration of hormone treatment (TAM or AI) plus OFS (H-OFS) and survival (aim 3).<h4>Design, setting, and participants</h4>This population-based cohort study included all premenopausal, early-stage breast cancer diagnoses between 2010 and 2020 in Alberta, Canada. Here, CYP19A1 is linked to breast cancer.