"Being on a par" means being equal in status or level. Opposite to this phrase, antonyms that can be used are:
1. Inferior- it means lower in rank, size, or quality.
2. Unequal- it means existing or occurring in an unequal or uneven manner.
3. Subordinate- it means lower in rank or position.
4. Inadequate- it means insufficient for a particular purpose.
5. Secondary- it means less important than something else.
These antonyms can be used to indicate a difference in status or level, indicating that one thing is below or above another.