I am having trouble understanding the basic concepts of ASN.1.
If a type is an OID, does the corresponding number get actually encoded in the binary data?
For instance in this definition:
id-ad-ocsp OBJECT IDENTIFIER ::= { id-ad 1 }
Does the corresponding 1.3.6.1.5.5.7.48.1 get encoded in the binary exactly like this?
I am asking this because I am trying to understand a specific value I see in a DER file (a certificate), which is 04020500, and I am not sure how to interpret it.
Yes, the OID is encoded in the binary data. The OID 1.3.6.1.5.5.7.48.1 you mention becomes 2b 06 01 05 05 07 30 01 (the first two numbers are encoded in a single byte, all remaining numbers are encoded in a single bytes as well because they're all smaller than 128).
A nice description of OID encoding is found here.
But the best way to analyze your ASN.1 data is to paste in into an online decoder, e.g. http://lapo.it/asn1js/.