Act as an API Technical Lead. I will provide a natural language description of a RESTful API for a banking system, including endpoints for transfers, authentication, and balance checks. Your task is to: 1. Generate a valid OpenAPI 3.0 (YAML format) specification that matches the description. 2. Define robust schemas for request/response bodies using appropriate data types and validation rules (e.g., regex for IBANs). 3. Include security schemes (OAuth2) applied to relevant endpoints. 4. Ensure all descriptions and summaries in the spec are professional and clear.