Selective attention supports top-down control by biasing information processing toward stimuli that are potentially relevant to the immediate goal. It has been recently proposed that theta band oscillations (~4-8 Hz) in the frontal midline regions are a key mechanism of endogenous selective attention. The current electroencephalography study investigated theta oscillatory dynamics using an inter-sensory cueing paradigm in which a symbolic cue indicated, on a trial-by-trial basis, the modality (visual or auditory) of the upcoming discrimination task. Time-frequency analyses were used to quantify phase- (evoked) and non-phase-locked (induced) fronto-central theta activity during preparatory attentional states. In a sample of 20 young adult participants, we found that those who relied on the cues to selectively attend to the sensory modality of the discrimination task performed more efficiently (i.e., faster and with greater accuracy) and presented greater non-phase-locked fronto-central theta power 200-400 ms post-cue onset. Moreover, greater non-phase-locked theta oscillations were associated with better behavioral performance. Secondary analyses on alpha oscillations revealed concomitant brain activity to theta with a pronounced decrease in alpha power in fronto-central regions, without significant effect on task performance. These findings suggest that increased non-phase-locked fronto-central theta oscillations are a neuronal correlate of preparatory attentional control and that the interplay of theta-alpha rhythms differentially contributes to attentional and perceptual aspects.