Can the caller to Allah be an oppressor during his call, what are the forms of that, and why did the Prophet (peace and blessings of Allah be upon him) single him out with this admonition?

The Prophet's counsel to Mu'adh ibn Jabal, may Allah be pleased with him, when he dispatched him to Yemen, was an admonition from the leader to his governors regarding the affairs of the Muslims, and a warning against oppression that might emanate from them. Oppression can issue from any person, no matter how great their status, and this includes taking the best of people's wealth or harming them with the tongue. The warning against oppression here is comprehensive of all its types. The reason for the warning against the supplication of the oppressed is that there is no veil between this supplication and Allah; rather, it is presented directly to Him, the Exalted.