public class OauthIntegrationController {

    public Static String accessToken = '';

    public static void oauthLogin(String loginUri, String clientId,
    String clientSecret,String redirectUri){


        HttpRequest req = new HttpRequest();

        req.setMethod('POST');
        req.setEndpoint(loginUri+'/services/oauth2/token');

        req.setBody('grant_type=authorization_code' +
      //  req.setBody('response_type = code' +
        '&client_id=' + clientId +
       '&redirect_uri='+redirectUri+
        '&client_secret='+clientSecret);

       /* '&username=' + EncodingUtil.urlEncode(username, 'UTF-8') +
        '&password=' + EncodingUtil.urlEncode(password, 'UTF-8'));*/

        Http http = new Http();

        HTTPResponse res = http.send(req);

        System.debug('BODY: '+res.getBody());
        System.debug('STATUS:'+res.getStatus());
        System.debug('STATUS_CODE:'+res.getStatusCode());
        JSONParser parser = JSON.createParser(res.getBody());

        accessToken = '';

        while (parser.nextToken() != null) {

        if ((parser.getCurrentToken() == JSONToken.FIELD_NAME) &&

        (parser.getText() == 'access_token')) {
        parser.nextToken();
        accessToken = parser.getText();
        }

        }

        restTest(accessToken );

        }


        public static String restTest(String token ) {

        HttpRequest req = new HttpRequest();

        req.setMethod('GET');
        req.setEndpoint('https://ap2.salesforce.com'+'/services/apexrest/TestRestResponseController');
        req.setHeader('Authorization', 'OAuth '+token);

        Http http = new Http();

        HTTPResponse res = http.send(req);

        System.debug('BODY: '+res.getBody());
        System.debug('STATUS:'+res.getStatus());
        System.debug('STATUS_CODE:'+res.getStatusCode());

        return res.getBody();
     }

public class oAuth_Controller{

private auth_response rt;

public pagereference auth_Step_1(){


    String auth_url = 'https://login.salesforce.com/services/oauth2/authorize';
    String params =
                        '?response_type=code' +
                        '&client_id=' + encodingUtil.urlencode('YOURCLIENTID','UTF-8') +
                        '&redirect_uri=https://na1.salesforce.com/apex/{YOURVFPAGE}' +                           '&prompt=consent' +
                        '&scope=' + encodingUtil.URLEncode('full refresh_token','UTF-8') +
                        '&state=step2';
        pageReference pr = New PageReference(auth_url + params);
        return pr;
}

public pagereference auth_Step_2(){

    if(apexPages.currentPage().getParameters().get('state') != 'step2')
        return null;


    HttpRequest req = new HttpRequest();
    Http http = new Http();

    String auth_url = 'https://login.salesforce.com/services/oauth2/token';
    String params =
                        '?code=' + apexPages.currentPage().getParameters().get('code') +
                        '&grant_type=authorization_code' +
                        '&client_id=' + encodingUtil.urlencode('YOURCLIENTID','UTF-8') +
                        '&client_secret=YOURSECRET' +
                        '&redirect_uri=https://login.salesforce.com/apex/YOURVFPAGENAME';

    req.setMethod('POST');
    req.setEndpoint(auth_url + params);

    HTTPResponse resp = http.send(req);

    rt = (auth_response)json.deserialize(resp.getBody(),auth_response.class);
    //Do something with the results
    return null;

}

public void getnewtoken(){

    HttpRequest req = new HttpRequest();
    Http http = new Http();

    String auth_url = 'https://login.salesforce.com/services/oauth2/token';
    String params =

                        '?grant_type=refresh_token' +
                        '&client_id=' + encodingUtil.urlencode('YOURCLIENTID','UTF-8') +
                        '&client_secret=YOURSECRET' +
                        '&refresh_token=' + encodingUtil.URLEncode(YOURREFRESHTOKEN,'UTF-8');

    req.setMethod('POST');
    req.setEndpoint(auth_url + params);

    HTTPResponse resp = http.send(req);


}



private class auth_response{

    public string refresh_token;
    public string access_token;

}
}

<apex:page controller="oAuth_Controller" action="{!auth_step_2}">
  <apex:form >
  <apex:commandButton action="{!auth_step_1}" value="Start oAuth" rerender="msgs"/>
  <apex:commandButton action="{!getNewToken}" value="Get new Token" rerender="msgs"/>
  </apex:form>

  <apex:outPutPanel id="msgs">
      <h1>Congratulations</h1>
      {!$CurrentPage.parameters.access_token}
  </apex:outPutPanel>
</apex:page>