Linear transformation of high-frequency electromagnetic waves in quasi-electrostatic waves provides a convenient way to heat overdense high-β plasma in prospective open traps. However, this process has a number of peculiar features which distinguish it from the case of a strong magnetic field, which is used for plasma heating in toroidal magnetic traps and other low-β devices, and from the case of Langmuir wave excitation in non-magnetized plasmas. This work presents a detailed theoretical study of these features.