Q.What are the assumptions made from ideal OP-AMP characteristics?

The ideal op-amp is an imaginary benchmark whose parameters are pushed to perfect limits so that a circuit's behaviour depends only on its external components. Its defining properties are: infinite open-loop voltage gain, infinite input impedance (it draws no input current), zero output impedance, infinite bandwidth, infinite common-mode rejection ratio, and zero input offset voltage. These lead to the two working assumptions used to analyse op-amp circuits: no current flows into either input, and (with negative feedback) the two inputs sit at the same voltage.

A practical op-amp such as the IC 741 comes close but cannot reach these limits. Its open-loop gain is very large but finite (of the order of $2\times10^{5}$); its input impedance is high but finite (about $2\ \text{M}\Omega$); its output impedance is small but non-zero (about $75\ \Omega$); its bandwidth and CMRR (about 90 dB) are finite. …