Pinyin "ying" has an "o" sound?
In IPA transcriptions of Chinese, ying is usually written as: [iŋ]. This is exactly like yin, except with an "ng" sound. However, as you noticed, some people pronounce it a little differently. This paper (page 9) transcribes the alternate pronunciation as [iɘŋ]. Wouldn't really call it an "o" sound, but the vowel is different than that of the yin.