There are several OS development related tutorials on this wiki. This page gives you a nice overview of tutorials that are around, based on subject area and difficulty.
These are "bare bones" tutorials that will land you with a basic kernel that's safe enough to use as a starting point for your own.
Bare Bones - Write a basic 32-bit kernel in C for x86
Meaty Skeleton - Template operating system
Setting Up Long Mode - Switching to long mode
Creating a 64-bit kernel - An introduction to 64-bit kernels
Real mode assembly bare bones - A tutorial series on writing a basic assembly language kernel
Pascal Bare Bones - A basic kernel in Pascal
Ada Bare bones - A tutorial on writing a basic kernel in Ada
FreeBasic Bare Bones - A basic kernel in FreeBasic
We also have bare bones for other platforms
GameBoy Advance Barebones - A tutorial on writing a basic GBA kernel
Sparc Barebones - A basic kernel for SparcStations
How to create a basic kernel in assembly
Babystep1 - Your first boot sector.
Babystep2 - Writing a message using the BIOS.
Babystep3 - A look at machine code
Babystep4 - Printing to the screen without the BIOS
Babystep5 - Interrupts
Babystep6 - Entering protected mode
Babystep7 - Unreal Mode
Babystep8 - 32-bit printing
Appendix A - Additional information
FlingOS Getting Started video series - A series of video tutorials giving a practical start to writing your first OS (aimed at x86, full examples available in each of ASM, C and C#)
Setting Up Paging - A tutorial that deals with setting up and maintaining a system with paging enabled
Setting Up Paging With PAE - As above, but with PAE enabled
Writing a memory manager - A tutorial on how to handle the RAM in a computer.
Writing A Page Frame Allocator - How to write a simple page frame allocator
Processes and Threads
Kernel Multitasking - How to create a kernel-space multitasking system.
Getting to User Mode- How to context switch into user mode.
Graphics & Video
Double Buffering - A handy way to prevent artifacts.
Bootable CD - A tutorial that explains how to create your own bootable CD
Bootable El-Torito CD with GRUB Legacy - A tutorial that explains how to create your own bootable GRUB CD
Rolling Your Own Bootloader - Describes what steps to take when you write your own bootloader.
Creating your own bootloader - The more advanced tutorial about creating bootloader.
Writing a bootloader - A basic tutorial about rolling your own bootloader.
Writing a bootloader for UEFI - If you want to roll your own UEFI bootloader, this will be helpful.
Writing GRUB2 Modules - A tutorial on writing GRUB2 modules. To add your own functionality to GRUB2.
Makefile - A guided demonstration of how you can use Makefiles
OS Specific Toolchain - A guide on adapting GCC and Binutils to your platform
CMake Build System - A guide demonstrating adapting KitWare's CMake Build System for building your operating system.
GCC Cross-Compiler - A guide that helps you create a compiler suited for OS development.
GDC Cross-Compiler - Same as the previous, but this time for the D programming language.
Executable File Formats
ELF Tutorial - A guide that details the process of loading ELF executables.
Manually Creating an ELF Executable - A guide that demonstrates how ELF binaries work, and how to build one from scratch using only a hex editor.
Porting Newlib - A guide on porting a common C library to your OS in progress
Using Libsupc++ - A guide on porting libsupc++ to get more out of the features of C++
Porting Python - A guide on compiling python for your OS