Find out if a resource defined in the <name>.yaml manifest already exists in OpenShift:
$ oc get --filename <name>.yaml
You can create or modify a resource in OpenShift using an oc apply command:
$ oc apply --filename <name>.yaml
If you create or modify a resource using oc apply, a kubectl.kubernetes.io/last-applied-configuration annotation will be attached to the resource. This annotation holds the last configuration applied to the resource. Following updates to the resource via oc apply will perform a three-way merge based upon the previously applied configuration, the provided input and the current configuration of the resource. This three-way merge provides you with a better control over resource modifications than a simple patch performed by an oc patch command. There is one caveat when using oc apply though: future changes to the same resource should be based off the last applied input file, i.e. you modify the last applied input file to fit your needs and reapply it. Otherwise you would be running into a danger of dropping the previously applied configuration.
Existing resources can be modified using an oc patch command (defaults to strategic patch type):
$ oc patch --filename <name>.yaml --patch "$(cat <name>.yaml)"
Resources that don't support strategic-mege patch can be patched this way:
$ oc patch --filename <name>.yaml --patch "$(cat <name>.yaml)" --type merge