We have derived the oxygen abundances of 50 F-type main-sequence stars, belonging to the Hyades open cluster, the Ursa Major group, and the field, using an NLTE analysis of the infrared triplet lines of O I at 7771-7775 AA. The stars of the middle F-range have previously been shown to exhibit a marked reduction in lithium abundance compared with both cooler and hotter objects in the same systems: the 'Li gap', for which several explanations have been proposed. Our results here show a substantial measure of uniformity in the oxygen abundances over the range in T(eff) between 5800 and 7400 K. The data might indicate a small dip (less than 0.1 dex) in the oxygen abundance for stars located in the Li gap. Microscopic diffusion seems to be the only known mechanism able to produce an oxygen dip. If this mechanism were also responsible for the Li gap, it would have to account for a depletion of up to two orders of magnitude in lithium and, at the same time, less than 0.1 dex in oxygen. We note, however, that turbulent mixing could modify a microscopic diffusion pattern in order to yield the observed abundances.