The following information is correct to the best of our knowledge for version 1.6.2 and on...
1) Go to the Forge website below and choose the version specified on the Orespawn version's Download Page.
Forge website:http://files.minecraftforge.net
2) Download the 'Installer' from the forge page you've chosen. You can choose adfly or direct, both work. The best place to save it is your desktop. It should be a .jar file.

3) After it has downloaded, double click the icon to launch it. If there is no error message, continue to step 5.
4) If you receive an error message saying, 'The directory is missing a launcher profile', just click Cancel for now. Next, click the Minecraft icon to load their launcher page. Simply log in! Close the game and double click Forge again to relaunch. Your error message should disappear.
5) Be sure to have 'Install Client' selected from the options in the forge window. The MC directory shown should be the one you use, but if it isn't you will need to find your directory and select it in the forge window. Click Ok.
6) A pop up will appear and tell you it was successful. Click Ok.
7) Done! Launch Minecraft again and where is says Profile: 'default', click the drop down and select 'Forge'. Then you can log in as normal and begin to play! note: you can now delete the Forge Installer from your desktop

The following information is correct to the best of our knowledge for version 1.6.4...
1) Go to the Forge website below and choose theversion specified on the Orespawn version's Download Page.
Forge website:http://files.minecraftforge.net
2) Download the 'Installer' from the forge page you've chosen. You can choose adfly or direct, both work. The best place to save it is your desktop. It should be a .jar file.

3) Open a new finder window. On the top bar click 'go' then 'go to folder'. A pop up will appear in the finder window. Type in '~/Library/'. Then click Go. Note: on Mountain Lion, you will need to hold option when you click go.
4) In the finder window select 'Application Support' then 'Minecraft'.
5) BEFORE step 6, we encourage you to copy all these files and put them in a new file on your desktop so that you have a back up copy of everything!
6) Hold the command key and select the following files:
options.txt
resourcepacks folder
saves folder
servers.dat
stats folder
7) Drag the above 5 files to your desktop for temporary storage.
8) Delete everything else in the Minecraft folder. Leave the finder window open and launch Minecraft. Log in when asked. It may take little while to load… Click Play!
9) When you reach the main screen, click quit game. Next, right click the Minecraft launcher and click quit.
10) Select the 5 files you previously dragged to your desktop and drag them back into the Minecraft folder that you left open. When asked, select 'apply to all' and 'Replace'.
3) Next, double click the Forge Installer icon to launch it.
5) Be sure to have 'Install Client' selected from the options in the forge window. The MC directory shown should be the one you use, but if it isn't you will need to find your directory and select it in the forge window. This directory MUST be the Minecraft folder in the Application Support folder. Click Ok.
6) A pop up will appear and tell you it was successful. Click Ok.
7) Done! Launch Minecraft again and where is says Profile: 'default', click the drop down and select 'Forge'. Then you can log in as normal and begin to play! (If another window pops up, just click play) note: you can now delete the Forge Installer from your desktop
EXTRA: When opening Forge, if you receive a message that says something like 'Could not open file because it is from a un identified developer' do the following:

Drag the forge file to your desktop. Right click it, and hit open. It should pop up with a message that says something to the effect of, 'Are you sure you want to open this?'. Click open, and you'll be asked to enter an admin username and password. After admin approval, it should open just fine.