Lords Prayer. Asking for Forgiveness For those who beleive in the doctrine of Once Saved Always Saved (OSAS).  How does one reconcile our need to not only confess our sins, but to ask God for forgiveness for those sins?  


  Forgive us our trespasses as we forgive those who trespass against us


Some might say that in Mathew 6 the word should be translated as "debts" which are not necessarily "sins." Yet the parallel passage in Luke 11 uses the word "ἁμαρτία" which clearly speaks of sin.

Question: 

If we have received forgiveness in full, and it is completed as many would suggest, why would the Lord have us petition (Pray) for our forgiveness when it has already, according to the OSAS doctrine, been freely given and can never be taken away?

This question are for the Faith Practices that adhere to 'assurance of salvation' which includes most but not all the ecclesial bodies that broke and continue to break away from the Catholic Church.
The issue you raised is rooted in the concept of justification. 

Through the fulfillment of the will of the Father in His Son, Jesus Christ, the Father declared us righteous in His sight. That is, the death and resurrection of Jesus Christ is our justification, the declaration of God that we are free of guilt and penalty of sin and acceptable to Him (Romans 4:25, NIV):


  He was delivered over to death for our sins and was raised to life for our justification.


Our justification also marks the beginning of sanctification, a continual process of being made holy by the power of the Holy Spirit, a lifelong process that makes us more and more like Jesus Christ.

We can argue that the Lord’s Prayer is the foremost declaration of our faith in the fulfillment of the will of the Father in His Son. Therefore, it is a means to justify ourselves to the Father. Via the Lord’s Prayer, prayed daily, we are led by the Holy Spirit in our daily lives and become more like Jesus Christ – the precise outcome of sanctification!