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We have conducted a deep survey (with a central rms of $55\mu\textrm{Jy}$)with the LOw Frequency ARray (LOFAR) at 120-168 MHz of the Bo\"otes field, withan angular resolution of $3.98^{''}\times6.45^{''}$, and obtained a sample of10091 radio sources ($5\sigma$ limit) over an area of $20\:\textrm{deg}^{2}$.The astrometry and flux scale accuracy of our source catalog is investigated.The resolution bias, incompleteness and other systematic effects that couldaffect our source counts are discussed and accounted for. The derived 150 MHzsource counts present a flattening below sub-mJy flux densities, that is inagreement with previous results from high- and low- frequency surveys. Thisflattening has been argued to be due to an increasing contribution ofstar-forming galaxies and faint active galactic nuclei. Additionally, we useour observations to evaluate the contribution of cosmic variance to the scatterin source counts measurements. The latter is achieved by dividing our Bo\"otesmosaic into 10 non-overlapping circular sectors, each one with an approximatearea of $2\:\textrm{deg}^{2}.$ The counts in each sector are computed in thesame way as done for the entire mosaic. By comparing the induced scatter withthat of counts obtained from depth observations scaled to 150MHz, we find thatthe $1\sigma$ scatter due to cosmic variance is larger than the Poissonianerrors of the source counts, and it may explain the dispersion from previouslyreported depth source counts at flux densities $S<1\,\textrm{mJy}$. This workdemonstrates the feasibility of achieving deep radio imaging at low-frequencieswith LOFAR.

Deep LOFAR 150 MHz imaging of the Bo\"otes field: Unveiling the faint low-frequency sky