The main differences between the Oberon-class and Porpoise-class submarines are as follows:

1. Hull Strength and Quietness: The Oberon-class submarines were a follow-up to the Porpoise class and featured a stronger hull and were even quieter. This improvement made them more durable and harder to detect underwater. Source

2. Hull Materials: Oberons employed QT28 steel instead of the UXW used in the Porpoise class. This new material was easier to fabricate and allowed for a significant increase in diving depth. Source

3. Fire-Control Systems: There were differences in the fire-control systems used between the Brazilian and British Oberons, with a Vickers system being fitted to the Brazilian boats. Source

4. Construction Materials: The construction of the casing for the Oberon class was of a glass fibre and alloy, which was an improvement over the Porpoise class. Source

5. Sonar Equipment: Both classes are easily distinguished by the characteristic bulbous dome at the bow, housing the sonar equipment. Source

6. Propulsion and Cavitation: There was a comparison between three and seven bladed propellers for propulsion and cavitation in both classes. Source

Here are some images of both classes for reference:

• Oberon-class:
• Porpoise-class: