Thank you for your question. The Chemistry Code does not make an exception to the Nemeth Code rule regarding omissions. NC Rule X, Sec.57 OMISSIONS states "... the omission symbol which is used must correspond to the sign which appears in ink print." In this case, that will be the oval. Your agency can make a decision to replace the omission shapes with a general omission symbol, if deemed appropriate, in which case you would need to state this substitution on the Transcriber's Notes page or in proximity to the problem.