Objectives: Impact of osteoarthritis (OA) on the initial treatment response of rheumatoid arthritis (RA) by treat to target (T2T) practice was compared between the patients with an onset age ≥65 years old (late-onset RA [LORA]) and those with an onset age <65 years old (young-onset RA [YORA]). Methods: A retrospective study was conducted on the patients with RA, who were referred to our clinic without treatment between January 2021 and July 2022. Patients with grade ≥3 OA according to the Kellgren-Lawrence (K-L) classification either in the knee or hand were classified in the OA(+) group and others were in the OA(-) group. The clinical data were compared at the diagnosis and one year after the initial treatment between the groups for 74 LORA and 59 YORA patients, respectively. Results: One year after starting treatment in the LORA patients, the OA(+) group had poorer disease activity control and greater disability in the several activities of daily living (ADL) than the OA(-) group. In the YORA patients, there were no differences in ADL disability between the groups. Conclusions: In the initial treatment of the LORA patients, the prevalence of OA was high, and impact of OA on LORA was larger than on YORA.