The C $1s$ and O $1s$ excitation of formaldehyde $({\mathrm{H}}_{2}\mathrm{CO})$ has been studied within an ab initio framework. The second-order algebraic-diagrammatic construction [ADC(2)] polarization propagator method has been used to calculate energies and oscillator strengths of the electronic transitions. For selected C $1s$ excited states also multireference configuration-interaction (MRCI) calculations were performed. The vibrational excitations accompanying the electronic transitions have been studied using a linear vibronic coupling model. The theoretical C $1s$ and O $1s$ spectra are in excellent qualitative agreement with high-resolution K-shell photoabsorption measurements. The present results support the previous assignments of the C $1s$ spectrum, while they revise the interpretation of the O $1s$ spectrum above 537 eV. In contrast to the C $1s$ case, the main photoabsorption intensity in the O $1s$ spectrum is due to nd rather than to np Rydberg excitations. For the two lowest singlet excited states, that is, the ${}^{1}{B}_{1}(\mathrm{C}1\stackrel{\ensuremath{\rightarrow}}{s}{\ensuremath{\pi}}^{*})$ single excitation and the ${}^{1}{B}_{2}(\mathrm{C}1s,\stackrel{\ensuremath{\rightarrow}}{n}{\ensuremath{\pi}}^{*2})$ double excitation, we find vibronic interaction with the ${}^{1}{A}_{1}(\mathrm{C}1\stackrel{\ensuremath{\rightarrow}}{s}3s)$ and ${}^{1}{A}_{2}(\mathrm{C}1\stackrel{\ensuremath{\rightarrow}}{s}3d)$ Rydberg states via the ${\ensuremath{\nu}}_{4}$ out-of-plane bending mode. In addition, the ${}^{1}{B}_{2}(\mathrm{C}1s,\stackrel{\ensuremath{\rightarrow}}{n}{\ensuremath{\pi}}^{*2})$ and the ${}^{1}{A}_{1}(\mathrm{C}1\stackrel{\ensuremath{\rightarrow}}{s}3s)$ states interact via the ${\ensuremath{\nu}}_{5}$ mode. The vibronic coupling leads to a complex spectral pattern in the low-energy part of the C $1s$ excitation spectrum, allowing one to interpret the finer details of the experiment.