Linseed oil sitting in a bowl, or spread on the surface of wood, is perfectly safe. But a linseed oil soaked rag provides a vastly increase surface area, so the oxidation happens faster, and the rag can provide insulation, trapping the heat. The increased temperature speeds up the oxidation even more, which raises the temperature even more, and the runaway feedback can quickly result in temperatures that will cause the rag to spontaneously burst into flame. This isn't one of those "do not drive car while sunscreen is in place" warnings. This is one of those "keep your finger off the trigger until you have the gun pointed at something you want to shoot" warnings. Rags soaked in linseed oil will catch fire, if you don't handle them properly, and they can do so far more quickly than you might think.
When the top is done, we want the edged MDF and the oak countertop to have exactly the same dimensions, and for their width to exactly match the width of the base.I could see three ways of doing this: 1, join the MDF to the countertop and use my belt sander to sand down their joined edges to match the base; 2, join the MDF to the countertop and use a hand plane to plane down their joined edges to match the base; or 3, use a flush-trim bit against a straight edge to route the MDF to the width of the base, then join the MDF to the countertop and use the flush-trim bit to route the countertop to match the MDF.
Begin by cutting off a 10-in. length of the board and setting it aside. Rip the remaining 38-in. board to 6 in. wide and cut five evenly spaced saw kerfs 5/8 in. deep along one face. Crosscut the slotted board into four 9-in. pieces and glue them into a block, being careful not to slop glue into the saw kerfs (you can clean them out with a knife before the glue dries). Saw a 15-degree angle on one end and screw the plywood piece under the angled end of the block.
This table top is also made from the hardest wood out of any manufacturer we reviewed: oak. In fairness, this can be both a blessing and a curse. The oak wood is extremely durable and noted for its lack of a porous nature. This means the top can withstand plenty of abuse from tool and workpieces. Moreover, you will not have to worry about liquids like lacquers and stains from being absorbed into the top. That said, the hard oak can also do a number on tool sharpness, dulling them prematurely if they are not carefully handled.