The purpose of this study is to evaluate the long-term safety and efficacy of ORADUR®-Methylphenidate in children and adolescents with ADHD.

In order to evaluate the long-term safety and efficacy of ORADUR®-Methylphenidate in children and adolescents with ADHD, subjects who participated in the previous OP-2PN012-301 study will be invited to join this extension study for 24-month follow-up. Subjects will continue to receive ORADUR®-Methylphenidate at the previously determined optimal dose.

Main Inclusion Criteria:

1. Subjects previously enrolled in OP-2PN012-301 study and completed 4-week study treatment
2. Both subjects and parents/guardians have provided their signed and dated informed consent form for the study

Main Exclusion Criteria:

1. Subjects who experienced unmanageable adverse events (AEs) after receiving ORADUR®-Methylphenidate
2. Subjects are taking a concomitant medication (ex: Monoamine Oxidase Inhibitor (MAOI) or other ADHD treatments) that is likely to interfere with safe administration of methylphenidate within 14 day prior to the study treatment initiation
3. Subjects are joining other clinical studies and receiving any other investigational medical products within 14 days prior to the study treatment initiation.
4. By the investigators' discretion, subjects with serious or unstable medical illness that will interfere with the evaluations of study efficacy and safety
5. By the investigators' discretion, subjects cannot understand or follow the instructions given in the study
6. Psychological, familial, sociological, or geographical condition potentially hampering compliance with the study protocol and follow-up schedule