To send an OAuth authenticated message using hardcoded consumer key / secret and access token / secret in iOS -- should be used judiciously, this may be a security risk -- we use the GTMOAuth library (https://code.google.com/p/gtm-oauth/).

iOS GTMOAuth Request

#import "GTMOAuthAuthentication.h"

GTMOAuthAuthentication *auth = [[GTMOAuthAuthentication alloc] 
    initWithSignatureMethod:kGTMOAuthSignatureMethodHMAC_SHA1 
    consumerKey:@"..." 
    privateKey:@"..."];
auth.accessToken = @"...";
auth.tokenSecret = @"...";
[auth setHasAccessToken:true];

// Send Request
if ([auth canAuthorize]) 
{
    // Setup request
    NSString *message = @"Hello world!";
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc] 
        initWithURL:[NSURL URLWithString:[NSString 
            stringWithFormat:@"https://api.twitter.com/1.1/statuses/update.json?status=%@",
                (NSString *)CFBridgingRelease(CFURLCreateStringByAddingPercentEscapes(
                    NULL, (CFStringRef)message,
                    NULL, (CFStringRef)@"!*'();:@&=+$,/?%#[]",
                    kCFStringEncodingUTF8))]]
        cachePolicy:NSURLRequestUseProtocolCachePolicy
        timeoutInterval:5.0];
    [request setHTTPMethod:@"POST"]; 
    
    // Sign & authorize request
    if (![auth authorizeRequest:request]) {
        NSLog(@"Error: could not authorize request.");
    } 
    else    // Authorized and ready to go
    {
        NSData *returnData = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil];
        NSString *returnString = [[NSString alloc] initWithData:returnData encoding:NSUTF8StringEncoding]; 
        NSLog(@"Response: %@", returnString);
    }
}