There is no obligation on an employer to use Occupational Health unless specified in the employment contract.
However, Occupational Health referrals are sometimes a sensible thing to do. As an employer you do not want to be making medical decisions about your staff.
Using Occupational Health is often relevant with a disabled employee. This is because they may require certain workplace adjustments which will allow them to do their job. An Occupational health referral would enable you to make the right adjustments. This helps to avoid discriminating against members of staff by unwittingly not implementing a reasonable adjustment.
If an employer is considering dismissing an employee due to their sickness record obtaining an Occupational Health report can be an important element.
Example
An employee has returned from an operation which will require a lengthy recovery period. The employee operates heavy machinery and wants to come back on reduced hours.
The employer decides to instruct an Occupational Health Advisor. The employer would then provide details to Occupational health and the specialist would arrange a consultation with the individual.
In this case the report is likely to say that the employee cannot safely work with heavy machinery. This would then mean that the employer could very reasonably refuse the request to come back to work. It may also lead to the employer deciding that it is necessary to think about dismissing the employer as they no longer are able to fulfil their role. The Occupational Health report would then be an essential feature of the capability dismissal.