Irina Shayk -- Half-Naked & Rebounding ... At Post-Oscars Bash

Think Cristiano Ronaldo's kicking himself yet??

Here's recently single Sports Illustrated: Swimsuit Edition supermodel Irina Shayk ... showing off everything from her underboob to her side ass at the Vanity Fair party Sunday night in Hollywood.

The 29-year-old gave Ronaldo the boot last month after rumors began circulating that he was bangin' other chicks ... so she's very single and by the looks of things, very ready to mingle.

For the record, there were a ton of hot single dudes at the VF party -- including Orlando Bloom, Jared Leto and Common.