First of all, the two brands were controlled by BBK in the past. However, due to internal reasons of the company, the subsidiary OPPO separated from the company and existed independently. Their relationship is that they are from the same school.
Oppo was founded first and took the high-end route, while vivo was founded later by the boss of BBK and took the mid- to low-end route.
Oppo and vivo are both brands that are transitioning from functional phones to smart phones. When it comes to product research and development, the two brands have different understandings. Oppo adopts a method that closely follows the development trend of the times. Oppo's products present a design concept with mainstream configurations and no particularly restrictive configurations. Keeping up with market development trends is their consistent R&D strategy.
vivo has adopted a completely different R&D approach than Oppo. Vivo has always been committed to differentiated R&D ideas. Vivo pays more attention to functional R&D and user preferences, and uses advanced technology to lead the development of the industry. Counting vivo's products, it is not difficult to find that from the SRS sound effect of the first-generation smartphone vivoV1, each of vivo's mobile phones has features that are ahead of similar products.
In terms of mobile phone photography, vivo is the leader in front-facing dual cameras, which can be said to have created a new era of mobile phone selfies. Oppo and vivo both focus on camera functions, and their latest two models both use dual-camera mode.
This answer applies to all OPPO models.