The headline above, old no!, is intended to be a pun on oh no! (Straits Times, 3 April 2010). While it may work for Singaporean speakers of English, this would probably be quite a stretch for speakers of most other varieties of English.
In British Received Pronunciation (RP), old no is pronounced /əʊld nəʊ/, and oh no as /əʊ nəʊ/ — so they are really quite different.
In Singapore English (SgE), a realistic standard pronunciation might be /oʊld noʊ/ and /oʊ noʊ/ respectively, assuming that a diphthong is more desirable than a monophthong in each word (incidentally, the diphthong /oʊ/ is usual in American English).
How do old no and oh no become rhymes in SgE? First, the final consonant cluster /ld/ in old is simplified, leaving [l]. And since SgE vocalizes or deletes dark /l/, we end up with [oʊ] for both old and oh.