Being pansexual means that you are attracted emotionally, sexually, or romantically to someone despite their gender.
Being omnisexual is similar to being pansexual, however one is not "gender blind," meaning that they don't take gender into account. Someone who is omnisexual does take gender/identities into account and may or may not have a gender preference.
Being bisexual means that you are attracted emotionally, sexually, or romantically to two genders, not necessarily the opposite sex.
Being biromantic is having only a romantic attraction to more than one gender.

The difference between pansexual and omnisexual is that pansexuals ignore people’s gender but omnisexuals dont