Sikhs are not meant to pray at Hindu temples because idolatory is strictly forbidden in Sikhism.
Sikhs might not go to Mosques but many Sikhs and Hindus do visit Muslim Dargaah’s (Muslim Samadhi-Mandir’s) but this is against Sikhism too..
Muslims who pray at Dargaah’s (tombs) and Hindus who wroship idols are both condemned in the Guru Granth Sahib.
Building Dargaahs (tombs) and praying at them is against Islam aswell but some Muslims have done this against our religion, they adopted it from Catholics etc. against the teachings of Islam.
The difference is Christianity or Sikhism do not teach vegetarianism.
In-Onkar comes from the word Om but like I said the meaning and symbolism behind the two is completely different.
The SIkh Gurus rejected the OM trnity, that’s why they add the Ik (one) prefix.