There is a chap on u-tube that says women should not say "Om" - they should say "am".
It is NOT a question of sect.The Smiritis and Puranas prohibit the utterance of OM for all who are not having the sacred thread.
In the commentary of sloka 29 chapter 289 of Santiparva of Mahabharata, Haridasa Siddhantabagish writes:
Shudras and females are not allowed to utter OM, because the Sruti (The Vedas) prohibits it ..सावित्रीं प्रणवं यजुरर्लक्ष्मीं स्त्री-शूद्रयोर्नेच्छन्ति इति श्रुत्या निषेधात्.[SAVITRIM PRANAVAM YAJUR LAKSHMIM STREE- SUDRAYOR NECHCHHANTI ITI SRUTYA NISHEDHAT]
Srmadbhagavam says (1/4/25) that "trayi" (which means both the A-U-M and the three vedas, should not be heard by women and the sudras.
In the Pranavakalpa of Skandapurana(Chapter 1, sloka 17-8), Lord Shiva says to Goddess Parvati : O Devi, being a female you are not allowed to utter OM.
Saints like Sri Ramakrishna(Reference:Sri Sri Ramakrishna-Leelamrita by Vaikunthanath Sannyal), Sri Sitaramdas Omkarnath(Reference-Pita-Putra by Sri Omkarnath) did not allow the sudras and women to utter OM.
And there are different religious restrictions on males also, especially on brahmins. No religion can accept 'Go as you like' and 'Do as your like' approaches until the aspirant attains Para-bhakti that frees him or her from all bondages and bindings