Oaths are to the Imperium through the person of the Emperor.
His published word is law.
Imperial legislation is literally whatever the emperor signs as law.
The only official check is that, if he's a danger, mad, etc, the moot can dissolve the imperium... or have him assassinated.
The practical matter is that the Emperor is limited by what the Nobles, Navy and Bureaucracy will enforce and promulgate.
An Emperor declaring in a moot session that henceforth all nobles will be succeeded by elected persons from within their sees probably wouldn't live long enough to matter, and the moot would promptly elect their new emperor, who would then repeal it, and life would go on as before. The news would probably only be passed that the emperor died of heart failure during the moot meeting.
Likewise, an emperor decreeing that henceforth all worlds will pay an MCr1 per head annual tax would find the nobles and navy unwilling to enforce same - because it would cripple their homes. The Emperor could declare it, and it would be law, but enforcement just wouldn't happen, as it would render the navy without anyplace to put in as the member worlds quickly went into massive rebellion. A Cr1 per head special annual assessment, however, would be an entirely different matter