This book presents 20 research and survey articles from several invited speakers and other participants at IWOTA 2023 held in Helsinki in August 2023. The topics range from concrete operators and matrices, such as Toeplitz, Hilbert matrix, integral, and Schrödinger operators, to more abstract operatory theory involving the invariant subspace problem, random vector norms, isometries, and quantum metric spaces, with further contributions related to inequalities for vector-valued functions, boundary value problems, fractional calculus, and the Korteweg–de Vries equation.