It would be impossible to list all the musicians he worked with in his career. However, the following musicians and artistes worked most frequently with him and for an extended period of time.
Assistant: G.S. Kohli
Music Arrangers: Sebastian D'Souza, Babu Singh, Dheeraj Kumar
String Instruments: Hazara Singh (Hawain guitar), Dilip Naik (Spanish guitar), Ustad Rais Khan (sitar), Ganguly (sarod), Zain Daruwala (sarod), Parasram (mandolin), Laximant (mandolin), Pandit Shivkumar Sharma(santoor), Pandit Ramnarain (sarangi). Violinists: too many to name.
Rhythm: G.S. Kohli (dholak), Abdul Karim (tabla), Ustad Zakir Hussain (tabla), Kersi Lord (ghungroo), Kawas Lord (bongo)
Key Instruments: Babu Singh (harmonium), Goody (accordian), Dheeraj Kumar (accordian), Balsara (organ), Sunny Castelino (piano)
Wind Instruments: Gangaram (clarinet), Manohari Singh (saxophone), Pt. Hariprasad Chaurasia (flute), Barve (flute)
G.S Kohli, also composed music for a few films independently and has some unforgettable songs to his credit, most notably of Shikari (1963). However, he continued to be OP Nayyar's assistant until the end. He died in 1994.
OP Nayyar is known to have often used a 70 piece orchestra for many of his compositions. 30 musicians in a song were common and then there were times when only 4 were used to create the same magic, when required.