Extracting a custom extension from a X.509 certificate by OID with OpenSSL

I have been able to extract a custom extension from a X.509 certificate by its index with:

X509_EXTENSION* ex = X509_get_ext(x509, extension_index);
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How do I extract the extension by its OID instead of its index?

Got it working with the following:

int my_nid = OBJ_create("1.2.3.4", "MyShortObjectName", "My Long Object Name");
int my_idx = X509_get_ext_by_NID(x509, my_nid, -1);
X509_EXTENSION* ex = X509_get_ext(x509, my_idx);