When a pilot says "We are cruising at 24,000 feet" that means 24,000 feet above sea level.
Alternatively the pilot could say, "We've been cruising at 28,000 feet but we are climbing to 30,000 feet so that we can clear Mount Everest by 1,000 feet." That would be 1,000 feet "above Everest", but at an altitude of 30,000 feet above sea level.
Mount Everest is 29,035 feet above sea level. If you were on the mountain it would not be "altitude" ,it would be "elevation", but would still be "above sea level".
I know what "altitude" means. You do not know what the OP meant by "above the ocean".Unless I am told otherwise by the OP, I assume that "above the ocean" means "over the ocean" as opposed to "above or over a land mass". And you are wrong about Everest - it is perfectly normal to refer to "altitude" up mountains. Mountaineers suffer from "altitude sickness" not "elevation sickness". "Elevation" refers to one method of describing the height of an object - be it a mountain or an aeroplane. "Altitude" is another.
Altitude in aviation, above the transition altitude, refers to a notional height above a notional sea level, not above the ocean. The actual elevation will vary from day to day and from place to place, depending on local barometric pressure. Mountains have an elevation because their size is constant. Mountaineers have an elevation - a position on the mountain. However, they are exposed to the physiological effects of altitude, which are driven by barometric pressure. Their elevation and their altitude are not necessarily the same.