Brown Field Cloud
If you already have a significant public cloud presence with another vendor, then you have already done a substantial amount of research and design work that can be re-used with Oracle. While OCI has some significant differences with the other clouds, in terms of the high-level architecture and design, there is a great deal of commonality. In fact, with the growth of deployment and configuration tools such as Terraform and Ansible that are designed to be multi-cloud, then even the tools that you used can be common.
In some areas, Oracle has some high-value services that are differentiated and would be beneficial to use, even in a multi-cloud environment - and we bring these to the fore in the appropriate areas within the framework.
If you already have an OCI tenancy that is in use but your organization, then we would advise you to review that tenancy against these best practices to establish whether it is a tenancy that could be scaled and built upon, or whether a new structure might be a better approach. OCI has a very flexible and mature set of features that allows existing resources to be logically moved, but in some cases, it might be easier to start afresh.
Note that starting afresh doesn’t necessarily mean a new tenancy - in many cases the best approach will be to create a new top-level structure so that the old and new structures can happily co-exist. This also gives the option of a later migration of the established resources into the new model one the new model has been implemented - this is the approach that we would generally recommend as it has minimal impact on existing resources.