Can I use an oat cookie crust for an apple pie?

Instead of a pie, maybe consider individual baked apples. Simply core and fill with the GF oat crumble, bake and serve. Top with some whipped cream and/or caramel sauce, sprig of mint and you don't have to worry about crusts, GF pie shells, runny pie after the first cut etc etc.

I don't know about the crust. I'd consider an alternate route - what about doing more of a Dutch apple pie using the GF pastry crust and the oak cookie as a crumble idea for the top? Or even switching to a crisp/crumble instead of a pie and losing the bottom crust entirely. I don't generally add a thickener to apple pie, the pectin from the apples is typically sufficient. If you want to sub a thickener I'd go with tapioca flour/starch and use half as much as you'd use of flour.