most OID's in our world start
Root->ISO(1)->Org(3)->dod(6)->internet(1)->private(4)

which is the 1.3.6.1.4

then you add in the company specifics and get something like

iso.org.dod.internet.private.enterpirises.cisco or 1.3.6.1.4.1.9

You can find a list of private enterprise numbers at ftp://ftp.isi.edu.in-note/iana/assigments/enterprise-numbers

BTW chimni has given excellent advice above in recommending you pick up Essential SNMP it cleared up a bunch of the mud for me