I have bought enough oak planks to get me to 66 construction (for gilded altar), and i was just wondering if Oak Larders are the fastest XP while using just oak planks.

I'd say carved oak dining tables (flatpacked) are faster. They don't require you to go into building mode and if you have the oak planks, you can just have your butler bring them to you while you keep making them.

I use the same, many people advocate larders since they use two more planks, but its not worth it. They're slower since you have to remove larder and enter building mode.