This thesis aimed to investigate cognitive and neural underpinnings of cognitive control in obsessive-compulsive disorder (OCD), focusing on suppression of thoughts and actions, cognitive flexibility, and habitual behaviour. Four experiments addressed hypotheses that, in comparison with appropriate control groups: (i) Patients with OCD are impaired in their ability to control actions as measured by the Stop-Signal/Go No-Go task; (ii) They also present deficits in attentional set-shifting in an extra-dimensional set-shifting task; (iii) OCD is marked by difficulties in the ability to control thoughts, as demonstrated by a Retrieval-Induced Forgetting (RIF) paradigm; (iv) Deficits in inhibitory control correlate with electroencephalographic markers, especially error monitoring and action tendencies; (v) Habitual and ritualistic actions in OCD are driven by both motor deficits and intolerance of uncertainty; (vi) Learning and practising a finger tapping sequence on a smartphone application (app) can have clinical benefits as a 'habit-reversal' treatment; and (vii) Metacognitive functions such as memory confidence and vividness are impaired in OCD, prompting the need to repeat actions. The thesis is structured in seven chapters, with experiments presented in chapters 3, 4, 5 and 6. Chapter 3 reports inhibitory deficits in a large group of OCD patients, showing impairments in action cancellation in this sample. These results are discussed alongside neural EEG markers and self-report measures, highlighting roles of error monitoring and enhanced action tendencies in the maintenance of OCD symptoms. Chapter 4 presents the results of a clinical trial conducted in collaboration with the NHS Highly Specialised OCD Clinic in Hertfordshire, where patients were randomised to either Treatment as Usual (TAU), a combination of Cognitive-Behaviour Therapy (CBT) and Exposure-Response Prevention (ERP), or Habit-Reversal Treatment (HRT). The latter consisted of a mobile phone application, and participants were asked to practise sequences of finger tapping movements. Participants were assessed at 3 timepoints (Baseline, Midterm and Endpoint). Results showed that HRT was equivalent to TAU in reducing symptoms, and indeed superior at enhancing quality of life in OCD. These results are discussed alongside neural markers and cognitive deficits in inhibitory control and extra-dimensional set-shifting. Chapter 5 presents data on a second group of patients with OCD and a matched control group, aiming to further clarify neural and cognitive dynamics of inhibitory control, error monitoring, and motor learning in OCD. For that end, both patient and control group were further separated into app and no-app training, enabling assessment of how the mobile application affects healthy participants, and whether the changes in OCD symptomatology seen in Chapter 4 were related to app training or to the passage of time. Electroencephalographic and behavioural data were collected at two different timepoints, separated by a month, to parallel the previous study and allow for comparisons. Chapter 6 further investigates inhibitory control deficits in OCD in an online study with yet another group of patients and control participants. The comparison between ability to control actions, as measured by the Stop-Signal Task, and thoughts, as per the RIF paradigm, showed significant RIF effects on controls, but not on patients, suggesting impaired thought inhibition in OCD. These results are discussed alongside a metacognitive memory test, which reveals the role of memory confidence as a possible cause of repetitive actions in OCD. A final Discussion (Chapter 7) brings together the findings of this thesis and considers their implications for the neuropsychological basis of OCD and its future treatment.