Hi Nikhil,
As far as I understand You will need either SAP MII or SAP PI as a middleman to establish communication between SAP ECC and OSI PI.
SAP MII-
SAP MII connects with OSI PI via SAP PCo(Plant Connectivity) agent and OSI PI too has ready connectors to integrate with SAP MII(via PCo).
SAP MII can call RFCs/BAPIs in any other SAP ECC Module(its SAP PM in your case).
In this manner your end to end communication will be established.
SAP PI -
SAP PI can connect with OSI PI as well as SAP ECC via WSDL.
I am not an expert of SAP PI hence others can contribute more.
P.S. - Kindly mark the answer as "helpful" or "correct" if it really serves your purpose.
Regards,
Muddassar