if efficient conversion of vegetable matter to oil is used. Forget olive oil, it is far too expensive. In fact, forget ANY edible plant product. Now for the inedible parts of the plant, including:

Forest floor debris and brush which is tinder for forest fires.
Farm waste: inedible plant residue.

With modern conversion into oil, about 2/3 of the energy of the feedstock comes out in the form of oil, the rest being used in the process or wasted. See Anything Into Oil for details.

Set up for maximum yield of (dry weight of) vegetable matter, a hectare yields at least 30 (metric) tons per year, equivalent to about 12 tons of oil after processing. (Yields can go much higher with Direct Irrigation, up to 100 tons per hectare per year.) Using this rough figure of 12 tons of oil per hectare/year, it is possible to derive some estimates. (By way of comparison, the average yield of palm oil per hectare is 4 tons per year, so I am not dealing in fairy-tale figures.)

The current cropland of the USA is about 180,000,000 hectares, the forest area about 225,000,000 hectares. I assume here that both can be harvested at about 10% of theoretical efficiency. Some crops produce relatively small yields, as a first approximation. Obviously, much work must be done to come up with more accurate numbers.

180+225 = 405, which I will round to 400.

400,000,000 * 10% = 40,000,000.
40,000,000 hectares * 12 tons/hectare/year = 480,000,000 tons.

See also Biomass Could Displace 30% of Petroleum Use This source gives an estimate of more than twice my very conservative estimate.