In Protocol 4.9, The P message consists of a pre-determined list of fields, listed at https://www.iqfeed.net/dev/api/docs/docs51/Level1UpdateSummaryMessage.cfm.
In later protocols, the default set of fields varies. However, the S,SELECT UPDATE FIELDS command lets you customize what fields are in the Q message, and this will apply to the P message as well. You just need to do a S,SELECT UPDATE FIELDS command, and include "Open Interest" as one of your field names. Then, when you watch symbols, Open Interest will be part of the message.
The command S,CURRENT UPDATE FIELDNAMES will tell you your current list of field names, whether it is the default set or if you have previously customized.
DTN IQFeed Implementation Support Specialist