Onigiri, Rice Ball
noun
握り is a "grip." When you add お it's elevated a bit. For some reason it becomes a rice ball (aka onigiri). To make a rice ball, you use your grip to make them into a ball shape, which is probably where it comes from.
おにぎり
The reading comes from 握り. Note that this is usually written in hiragana as おにぎり.
私は息子がコンビニのお握りを食べることを禁止しました。
I forbade my son from eating convenience store rice balls.