The first step is to connect the cables from your Bus Pirate or any standard UART cable to the Oak, I had a bus Pirate handy so I used this, which adds some extra steps, but others may find the general info useful

in the photo example of the Bus Pirate the wires I have used are as follows

• Grey – Ground
• Blue – MOSI
• Purple – MISO

• Grey – GND
• Blue – RX
• Purple – TX

• Open terminal. I use Putty
• Set Connection type to Serial
• Click on the Serial options link bottom left of Putty and set to the COM port your Bus Pirate is on (You can find out this info in Device manager in windows under the Ports Com & Lpt section)
• set the baud rate to 115200, Data bits 8, stop bits 1, parity none, Flow control none and hit open
• entering i will give you some basic info
• enter m to change mode
• enter 3 for Uart
• enter 10 for BRG Raw value we want one not specified in the list (74880) which equates to 52
• enter 1 for Data bits and parity (8, NONE *default)
• enter 1 for Stop bits (1 *default)
• enter 1 for Receive polarity (Idle 1 *default)
• enter 2 for select output type (Normal (H=3.3V, L=GND))
• Now you need to change the baud rate of the Bus pirate to match, as you are just going to pass thru data as it wont keep up otherwise
• enter b to set baud rate
• enter 10 for BRG Raw value we want one not specified in the list (74880) which equates to 52
• after you enter 52 it will ask you to adjust terminal to match, so Right click on the top of the putty window and click change settings
• Change Speed (Baud) rate to 74880 in the the Connection/Serial section and press apply
• enter (1) to change the Bus Pirate in to UART Bridge mode and press y to confirm
• now power cycle your oak and should get something in the terminal like this

ets Jan 8 2013,rst cause:2, boot mode:(3,0) load 0x40100000, len 3632, room 16 tail 0 chksum 0xc0 load 0x3ffe8000, len 352, room 8 tail 8 chksum 0x82 csum 0x82 OakBoot v1 - N,BC,8