Have a 3' X 7' Redwood live-edge slab. Sanded down to 220 grit. After doing some research I decided to coat, multiple light coats, of Danish Oil. It looks great in terms of grain/knots etc but very "flat" or dull. What can I do to make it shine like some you see on the internet?
"Danish oil" type finishes are generally penetrating finishes, a dilute mixture of oil and varnish (often/usually weighted towards the oil) designed to sink in and give an in-the-wood look, so will inherently give less gloss than finishes that build on the surface, i.e. film finishes such as shellac, lacquer or varnish. But assuming the wood surface is flat and very smooth you can still get a decent shine with them, with finer sanding (often beyond 220), multiple coats1 and lots of rubbing. But to get a truly glossy surface with a finish like this you generally need to be working with finely sanded hard woods2.
Penetrating oil finishes are not generally water-resistant enough for a working bar or countertop. If it's mostly for show however it could be fine, but you'll want to use coasters to be on the safe side.
For proper waterproofing you need to use varnish at least, or a dedicated 'bartop finish' which are often poured on the wood and form a very thick, very glossy surface film. A little more in this previous Answer, Protecting against water damage.
For more on wiping varnish, including Bob Flexner's instructions on how to make and apply it, see how to fix very light scratches in polyurethane finish?
1 Difficult to generalise how many, but let's say more than three or four and as many as six or seven. But do note results depend greatly on the "Danish oil", they can and do vary from make to make with some being thinned less and containing more varnish and these are the ones that will give a superior gloss with fewer coats, more varnish = better gloss.
2 Harder woods generally give a superior gloss without much effort compared to softer woods. You can see this effect in the flesh any time you're working with pine that has knots in it, by noticing how the knots (which are usually very dense and hard) easily end up glossier than the surrounding normal wood...... sometimes before any finish has even been applied.