String content = '{"Name": "Some acoount", "Phone":"9742882954"}';
 HttpRequest req = new HttpRequest();
 req.setEndpoint('https://ap1.salesforce.com/services/apexrest/Accountacc');
 req.setMethod('POST');

 String username = 'sarvesh01@gmail.com';
 String password = 'parvati@3';

 Blob headerValue = Blob.valueOf(username + ':' + password);
 String authorizationHeader = 'BASIC ' +
 EncodingUtil.base64Encode(headerValue);
 req.SetBody(content);
 req.setHeader('Content-Type', 'application/json; charset=utf-8');
 req.setHeader('Authorization', authorizationHeader);
 req.setHeader('Authorization', 'OAuth '+UserInfo.getSessionId());
 Http http = new Http();
 HTTPResponse res = http.send(req);
 System.debug(res.getBody());

@RestResource(urlMapping='/Accountacc/*')
global with sharing class callAccount {

@HttpPost
 global static String doPost() {

 RestRequest request = RestContext.request;
 RestResponse response = RestContext.response;
 String jSONRequestBody=request.requestBody.toString().trim();
 Account accObj = (Account)JSON.deserializeStrict(jSONRequestBody,Account.class);
 insert accObj;
 return accObj.Id;
}
@HttpGet
global static Account doGet() {

 RestRequest request = RestContext.request;
 RestResponse response = RestContext.response;
 String accountId = request.requestURI.substring(request.requestURI.lastIndexOf('/')+1);
 Account acc= [SELECT Id, Name, Phone, Website FROM Account WHERE Id = :accountId];
 return acc;
}
}

HttpRequest req = new HttpRequest();
req.setHeader('Authorization', 'OAuth ' + accessToken);
...