Do you mean traditional oxford cloth or the thinner pinpoint oxford? Or do you want patterned broadcloth?
If it's in your budget, Mercer & Sons make the best traditional OCBD out there today. Each shirt is made to order - plan to budget over $100 for each one after you add on various options. Their broadcloth fabrics are also very nice.
My second choice would be J.Press. I think the prices are around $90 now...I've had several for over 4 years and they hold up very well. Wait for their bi-annual 25% off sale if you want to save a few bucks.
Third choice would be Brooks Brothers, but make sure you get the USA made ones. If you can get your hands on a corporate discount card, you can get them for 3 for ~$180. They also offer slim fit (and maybe extra slim?) if you're the thin type.
Budget choices:
Polo Ralph Lauren @ TJ Maxx and Marshalls. The Custom Fit shirts are for thin guys, I prefer the traditional fit. Usually you can pick these up for $25-40 each. I'm wearing one right now in fact.
Lands End. The prices hop all over the place depending on what sale they're running. Stay away from the non-iron shirts - they don't breath well (the same is true of any non-iron shirts). The Hyde Park model is my favorite...it's a thicker traditional oxford cloth rather than the thinner cheaper oxford cloth they offer or pinpoint or broadcloth. It actually breathes very well in the hot summer too, better than pinpoint or broadcloth (which is true generally also). The downside to the shirts is the fit. The traditional fit is slim in the waist but broad in the chest - almost an athletic cut. The tailored fit models are quite slim and for thin guys only.
Lastly, make sure you get shirts that come in proper neck and sleeve sizes. The s/m/l type ones are never as good and are usually cut weird.
Another tip: nothing is better for getting stains and collar ring out of shirts that Octagon borax soap. Pre-treat with that, wash your shirts yourself and hang dry. No starch when ironing and your shirts will last a lot longer.
How do I know all this? This is what happens when you've attended a big name east coast law school and have to look the part (old money/prep school/young republican) for interviews and jobs.