Background: Breast cancer (BC) in reproductive-age women raises concerns about fertility preservation, particularly as systemic therapies may compromise ovarian function. Evidence on whether tumor immunohistotype influences ovarian reserve and fertility preservation outcomes remains limited. Methods: We conducted a prospective cohort study of BC patients referred for fertility preservation counseling between November 2020 and May 2025. Ovarian reserve was assessed using anti-Müllerian hormone (AMH) levels and antral follicle count (AFC). Controlled ovarian stimulation (COS) and oocyte cryopreservation were performed according to standardized protocols. Patients were stratified into triple-negative BC (TNBC) and hormone receptor-positive (HR+) and/or HER2+ groups. The primary endpoint was ovarian reserve differences by subtype; the secondary endpoints were ovarian response and oocyte yield. Results: Of 358 patients, 152 were enrolled, and 139 (91.4%) underwent COS, for a total of 145 cycles. The median age was 33 years, median AMH 5.4 ng/mL, and median AFC 17. No significant differences were observed between the TNBC and HR+/HER2+ groups in AMH, AFC, oocyte yield, or mature oocyte rate. Sub-analysis revealed a significantly lower mature oocyte yield in luminal-B tumors. Conclusions: Ovarian reserve and cryopreservation outcomes appeared preserved in TNBC compared with those in patients with HR+/HER2+ BC at diagnosis. These findings provide reassurance that baseline fertility potential is not compromised by tumor immunohistotype.