Oklahoma...Can I claim my child for tax purposes on my ex's year if he has failed to pay any child support in over 3 years?

First, keep in mind the typical writing of this provision when it refers to 2021 or an odd year, will be referring to the tax year and not the time when the claim is made and return is filed.  So 2020 is an even year, but it is when one files their 2019 taxes and would be considered an odd year (tax year).  A provision setting a condidtion precedent that current support is all paid for the obligor to be able to utilize the dependent deduction is very common.  Not having it in the decree would make me ask him in writing if he would agree to forgo that provision, if you had to even say, lets figure out the value of the dependent deduction to each of us, and the one for whom it has a greater value should be able to claim it, you could even entice him with offering credit toward his child support obligation.  If that doesn't go anywhere, file to enforce support and to modify the Decree with the addition of that provision.  Both are straight forward and logical so they won't typically lead to protracted fighting in court. The easiest case for enforcement is one where you can point to things he has purchased that were NOT court ordered, where he used his income instead of paying the child support.  An Application for Contempt Citation is the most common way to get his attention, it is a "quasi-criminal" proceeding, if he fails to appear at time of arraignment, the Court typically authorizes a Bench Warrent with Cash Bond set in an amount he will have to pay in order to be released from County Holding.  If he is employed it would be a good idea to get an "Income Assignment" put into place. This would all be much easier to accomplish with a lawyer, you may be able to be awarded at least a portion of your attorney fees too, unless he has a good excuse for only paying less than $2 on average for each of the last 42 months.  As you may be aware lawyers are NOT allowed to represent parties in family law matters with a "contingent fee" as it is feared that it will make litigation even more contensious and never-ending, but on enforcement of an order to collect payment a contingent fee is allowed.  I'm not sure if you'll find any lawyers willing to take on the case on a contingent fee, but it may be an option.  DHS-Child Support Enforcement is very overworked and underfunded, understaffed, etc. but it wouldn't hurt to pay the nominal fee to open a case with them, they can suspend his state issued licenses as a motivation to get current on his payment of child support. Call a lawyer, many will give you a FREE initial consultation.