If a commander is phased out, do I still control it?
702.26b If a permanent phases out, its status changes to “phased out.” Except for rules and effects that specifically mention phased-out permanents, a phased-out permanent is treated as though it does not exist. It can’t affect or be affected by anything else in the game. A permanent that phases out is removed from combat. (See rule 506.4.)
Example: You control three creatures, one of which is phased out. You cast a spell that says “Draw a card for each creature you control.” You draw two cards.
Example: You control a phased-out creature. You cast a spell that says “Destroy all creatures.” The phased-out creature is not destroyed.
You will not get to choose both modes of Jeska's Will.
One way to get a better understanding of the rules and meanings of words in Magic the Gathering, is to look up their definition. In this case, one of the common Wiki's says this about Phasing:
Phasing is a mechanic where permanents may phase out, causing them to be treated as if they don't exist, until they automatically phase back in on their next untap step.
As such, if your commander is phased out, due to the oubliette, the game treats this as if the commander doesn't exist. So, you can't control it (it doesn't exist) and so you don't get to choose both modes. You have to pick one mode in Jeska's Will.