Correct spelling for the English word "observed of" is [ɒbzˈɜːvd ɒv], [ɒbzˈɜːvd ɒv], [ɒ_b_z_ˈɜː_v_d ɒ_v] (IPA phonetic alphabet).
The phrase observed of does not have a specific etymology as it is a combination of two common English words. Observed comes from the Latin word observare, meaning to watch, keep an eye on. Of comes from Old English and has various uses, including indicating possession or origin. When used together, observed of means watched or noticed by.