Amat, who resigned as finance and health minister on May 6, began his second term as the Assembly Speaker today.
Earlier, he was Speaker of the Assembly from May 25, 2009 till May 20, 2014.
Amat's election came a day after the ruling BJD moved a notice for the Speaker's election.
Niranjan Pujari had tendered his resignation as the Speaker on May 5 ahead of the ministry reshuffle on May 7.
Deputy Speaker of the Assembly Sanand Marandi chaired a brief session of the Assembly where Chief Minister Naveen Patnaik moved a motion and proposed Amat's name for the Speaker's post which was seconded by Parliamentary Affairs minister B K Arukha.
Mishra said, it would have created a better tradition had the government consulted the opposition before announcing Amat's name for the post.
The Congress leader also hoped that the new Speaker would work in a neutral manner and protect the opposition's interest in the House.