Arcs are delegated hierarchically: 1.2.840 is the US branch under ISO, 1.2.840.113549 belongs to RSA Security (PKCS), 1.3.6.1.4.1 holds private enterprise numbers, 2.5 is the X.500 directory and 2.16.840.1.101.3.4 covers NIST algorithms. In X.509 you meet them constantly: 1.2.840.10045.2.1 (id-ecPublicKey), 1.2.840.10045.3.1.7 (P-256), 1.3.101.112 (Ed25519), 2.5.29.17 (subjectAltName), 2.23.140.1.2.1 (CA/Browser Forum domain-validated policy).
DER encodes an OID with tag 0x06: the first two arcs combine as 40 × first + second, and each later arc is written in base 128 with the top bit as a continuation flag. 1.2.840.10045.2.1 becomes 06 07 2A 86 48 CE 3D 02 01.
Unknown OIDs are not errors in themselves: certificate parsers show them as dotted numbers, and a critical extension with an unknown OID must cause rejection (RFC 5280 §4.2). The ASN.1 explorer names the well-known ones.