If your splunk search reads
index=_internal | table host
And while configuring the alert, if you enter your host name to be $result.host$, Splunk would configure the trap to take the value of host in the search result to assign it to host name. $result.$ is only to specify how the resulting fields should be specified.
The OID is usually specified by the trap receiver. Per the link you shared, if the OID for Splunk Enterprise is "126.96.36.199.4.1.27389.1.1", for the SNMP Splunk Modular Alert configuration, set the
Enterprise OID to "188.8.131.52.4.1",
Specific OID to "27389" and
Specific Trap ID to 1.
... View more