The following symbols, which kind of look like they contain circles, are covered in Section b.7: Curved Lines: $\infty$ $\propto$ $\partial$. The greek letters $\delta$ $\theta$ $\Theta$ $\rho$ $\sigma$ $\phi$ $\Phi$ are covered in Section b.8: The Greek Alphabet.

This is the number zero, or possibly the letter O.

The percent sign is a short-hand way of saying $/100$ or ‘for every hundred’. See Section 4.2.6: Percentages

The degree symbol can indicate an angle (Section 10.3: Angles), or a temperature (Section 2.1.2: Temperature).

Function composition is explained in Section 8.2.3: Composition of Functions.

This is the symbol for contour integration, which is not covered in this book.

This is the symbol for the empty set (Section 8.1: Sets). It is easy to confuse with the greek letter $\phi$ (Section b.8: The Greek Alphabet). Sometimes people write the number zero in a similar way.

$\oplus \otimes \ominus \oslash \odot$

Circled versions of the operators $+$ $\times$ $-$ $/$ $\cdot$ are sometimes used in abstract algebra to represent something very similar to the usual operators. For example, $\oplus$ might be used for modular addition.