Here’s an example of a letter you could write:

Your name and address
Landlord’s name and address
Date

Dear landlord,

I am writing this letter to request an immediate return of my security deposit of $ amount.
By law I was entitled to receive either a full refund of my deposit or an accounting of what the deposit was used for within 31 days from when I moved out. I moved out on date and I have received neither my deposit nor the accounting.  Therefore, by law (ORS 90.300) I am now entitled to twice the amount that was wrongfully withheld, which totals $ twice the amount of withheld deposit.

Please respond to me within the next amount of time with your plans to return my deposit. If I do not hear from you by that time, I will pursue legal options to recover the $ twice the amount of the withheld deposit I am entitled to under Oregon law.

Sincerely,

Your Name