You can watch One Punch Man Season 1 on several platforms including Crunchyroll, Hulu, Amazon Prime Video, and Netflix. It is available for streaming on Crunchyroll and Hulu, and you can buy or download it on Amazon Video, Apple TV, and Google Play. Additionally, One Punch Man Season 1 is legally available for free on YouTube on the Muse Asia channel in some regions. Here is a summary of options:

• Crunchyroll (streaming)
• Hulu (streaming)
• Amazon Prime Video (streaming and purchase)
• Netflix (streaming, availability varies by region)
• Google Play (purchase and download)
• YouTube Muse Asia channel (free legal streaming in some areas)

This gives you flexibility depending on your location and preferred service.