Can Oz Yashir facilitate forgiveness only during pesukei dezimra or at any time its being said?
The idea that saying Oz Yashir has the power to facilitate forgiveness is based on the Gemora that a person that had a miracle done for them, realizes it, and sings shira as result is considered like a new creation. This new creation is now new and without sin. Therefore, in order to merit that forgiveness, one has to say Oz Yashir with a feeling as if the person just walked thru the Red Sea, and it split before his very own eyes… If the person can say it with the enthusiasm as if it was just experienced, then the person merits forgiveness. The sefer Chareidim adds to this that when a person says it every day enthusiastically, that it is surely like it was said when they actually crossed thru it. Therefore, try saying it while picturing it as if you just went thru it, and say it as best and happily as you can every day and IY”H it will have its proper power and achieve forgiveness. As far as I have seen it does not have to be said specifically during pisukei dzimra.
Sefer Chareidim 73, M:B 51-57.