Background: Patients with isolated medial compartment osteoarthritis requiring surgical intervention generally have two surgical options: unicompartmental knee arthroplasty (UKA) and proximal tibial osteotomy (PTO). Outcomes of reoperation rates and survivorship are important for counseling patients on treatment options. Methods: A retrospective, comparative cohort study was performed for a consecutive series of patients in the Military Health System who underwent either UKA or PTO between 2003 and 2018. All patients were between 18 and 55 years old and diagnosed with isolated medial compartmental arthritis. Cases with concurrent meniscal or cartilage procedures were included, while cases with concurrent ligament reconstruction were excluded. A minimum 2-year follow-up was required. The primary outcome was conversion to total knee arthroplasty, and the secondary outcome was reoperation for any reason. Results: A total of 383 procedures were performed for isolated medial compartment arthritis in 303 patients (UKA 270, PTO 113). A multivariate analysis showed that PTO was associated with decreased risk of conversion to TKA compared to UKA (P = .0364). However, the reoperation due to complications was significantly higher in the PTO group (21.2% vs 2.2%; P ≤ .01). The 5-year conversion rate was 13.7% for UKA and 3.5% for PTO (P = .0033) with an average time to conversion of 3.1 years for UKA and 2.9 years for PTO (P = .7805). Conclusions: In young patients with isolated medial compartment arthritis, conversion rates to TKA are higher with UKA compared to PTO. However, overall reoperation rate is higher with PTO, secondary to complications and revision procedures. Overall survivorship is acceptable for both procedures.