Use an material for the o-ring which is compatible for both water and the type of oil you’re using.

one design I just ran across this morning on one of our exhibits (designed to hold a vacuum, but that’s just the inverse of what you want to do ):

A cap with a clearance fit into your tube, a plug with a gland for an o-ring, and use three screws to squeeze them together and expand the o-ring against the tube.
o-ring_end_plug.pdf