The most common plural of opus in English is opuses. Some people use the Latin plural, opera. Opi is fairly common in the field of classical music, though mostly in informal contexts. The use of any of these three pluralizations may result in the speaker being corrected, though opi, above all, should be avoided in formal contexts.
Where did you pull that from. Opi is not used within classical music. Why do you think they call an opera, an opera?