All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If you call your aunt ‘O my mother’ as a way of respecting her, then this is permissible. However, you should not call your mother ‘O my aunt’ as this may be considered as degrading her status, and this may lead you to not fulfill your obligations of being kind and dutiful to her as your mother; so you should be careful.
For more benefit, please refer to Fataawa 90201 and 92093.
Allaah Knows best.