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Actually $\mathcal O$ is some standard point and $r$ is where potential depends on. $$V(b)-V(a)=-\int_{\mathcal{O}}^B \vec E \cdot d\vec l +\int_{\mathcal{O}}^A \vec E \cdot d\vec l $$ $$ =-\int_{...
Answer
#1: Initial revision
Actually $\mathcal O$ is some standard point and $r$ is where potential depends on.
$$V(b)-V(a)=-\int_{\mathcal{O}}^B \vec E \cdot d\vec l +\int_{\mathcal{O}}^A \vec E \cdot d\vec l $$ $$ =-\int_{\mathcal{O}}^B \vec E \cdot d\vec l-\int^{\mathcal{O}}_B \vec E \cdot d\vec l$$ $$=-\int_A^B \vec E \cdot d \vec l$$
which was completely related to OP's equation. But the problem was the OP was talking about a potential point while it's valid for potential differences.