What are the differences between Hindu OM and IK ONKAR in Sikhism?

Respected Aarushi Ji;
Thank you for asking a good question to clarify your thoughts.
There is no such comparison between OAM and IKK OANGKAAR, in Bani enshrined in Sri Guru Granth Sahib.
I am not a scholar of Hinduism so cannot fully and accurately answer the meaning of OAM to a Hindu. However, from what I know OAM stand for trinity: Brahma, Inder, and Shiva,
The ੴ represents ONENESS of it all. Only One Creator, sustainer, and destroyer and present in this Creation itself.
OAM is external, (sought outside one’s self), for a Hindu and ੴ is internal (discovered within oneself) for a Sikh.
And there are many more differences except for a non-Sikh to comprehend it, they have to unlearn what they know and look at it in absence of any preconceived ideas.
Hope this clarifies it for you.