opensearch cost increase drastically in last two weeks from $5.76 to $41.99 per day

Hello.

Try setting the filter in Cost Explorer as shown in the image below to see the breakdown of OpenSearch charges.

Please check how much OCU is being used.
I think the fees will increase as more searches are performed using the knowledge base.
https://aws.amazon.com/opensearch-service/pricing/?nc1=h_ls#Amazon_OpenSearch_Serverless

here is the screenshot for the same. we are not using heavy searches too. its just less than 50 searches per day

Thank you for sharing the image.
Looking at the image, it appears that most of the charges are due to OCU related to search.
At what level is OCU set?
I think you can check the amount of OCU set from the OpenSearch console.

It is set at 10 OCU. But to whatever level it is set, how do i know what is causing the cost increase ?

it is currently showing at 6 OCU that there is no search at all, is this based on volume of embeddings with the search ?

As you can see in the documentation, you will be charged $0.24 per hour per OCU used for searches.
Therefore, if the amount of data searched increases, the number of OCUs used will also increase, so I think the price will increase.