Oxford shirts are considered casual.
https://m.thebay.com/polo-ralph-laur...E&gclsrc=aw.ds
Vs dress shirt
https://ca.suitsupply.com/en_CA/clas...tblue&pdp=true
Both shirts are similar in style in colour. However, notice how the Oxford shirt has button down collars vs a regular collar with collar stays. The Oxford material is also thicker cotton than the dress shirt.
Realistically, you can wear either style in an office in North America and to most people, they both look like "dress shirts". But as I was corrected earlier, if you truly want to give off a casual vibe, while still looking put together, go for the Oxford shirt.