[SalesForce] System.HttpResponse[Status=Unauthorized, StatusCode=401] from Apex Controller

I have the following code, that retrieves picklist values dependent on a record type:

public static List<String> getPicklistValuesByRecordType(String objectType, String recordTypeId, String fieldName) {
    //Endpoint
    system.debug('objectType : ' + objectType);
    system.debug('recordTypeId : ' + recordTypeId);
    system.debug('fieldName : ' + fieldName);
    String endpoint = URL.getSalesforceBaseUrl().toExternalForm();
    endpoint += '/services/data/v41.0';
    endpoint += '/ui-api/object-info/{0}/picklist-values/{1}/{2}';
    endpoint = String.format(endpoint, new String[]{ objectType, recordTypeId, fieldName });
    EncodingUtil.urlEncode(endpoint,'UTF-8');

    //HTTP Request send
    HttpRequest req = new HttpRequest();
    req.setHeader('Authorization', 'OAuth ' + UserInfo.getSessionId()); 
    req.setEndpoint(endpoint);
    req.setMethod('GET');

    system.debug('endpoint: ' + endpoint);
    Http http = new Http();
    HTTPResponse res = http.send(req);
    system.debug('res: ' + res);
    //Parse response
    List<String> result = new List<String>();
    Map<String,Object> root = (Map<String,Object>) JSON.deserializeUntyped(res.getBody());
    if(!root.containsKey('values')){ 
        return result; 
    }
    List<Object> pValues = (List<Object>)root.get('values');
    for(Object pValue : pValues){
        Map<String,Object> pValueMap = (Map<String,Object>)pValue;
        result.add((String)pValueMap.get('value'));

    }
    System.debug(result);
    return result;
}

When i run the following code from Execute Anonymous Window, i receive

System.HttpResponse[Status=OK, StatusCode=200]

But when running it from apex code that's being called by a component, i receive

System.HttpResponse[Status=Unauthorized, StatusCode=401]

Maybe its because i can't get UserInfo.getSessionId() from the code?

Please advise.