It's not the "오" that's different from "요", but rather "오" is part of "십시오". 십시오 is from another speech style. I wouldn't worry about it at all (at least not anytime soon)^^ Just recognizing it

oh, Sweetheart, I think you don't really have to worry about this issue, Because there Korean Grammar law has changed on this issue in 1988. we still pronounce ~~이요 but they say we have to write 오 in that based on the law. so, I am 100% sure that most of Korean are also very very confused on this question. when you write, 어서오십시오 is right, but we pronounce 어서오십시요. Actually 하오체, 하오style is gone now, i think you never gonna use 하오 style unless you watch Korean old history dramas or read old Korean novels.

1. 조심하세요 2. 조심하십시오 하십시오 is the imperative form of 합니다 and 하십니다 (honorific -시- added to 합니다 for higher politeness). This is commonly called formal-polite style (technically "하십시오체" or "합쇼체" in Korean speech level designation), which shows greater respect than the informal-polite style of 조심해요 and 조심하세요. This -시오 as well other obscure -오 sentence endings are traditional ending forms distinct from "-요" which is a simple suffix for politeness. In modern times, the 하십시오 style is not used much except in very formal occasions like public address (TV, radio, conferences). The shorter and simpler style of -요(technically called "해요체") dominates in day-to-day conversations. TV programs may be the best place where one can see the -오 ending commonly used (they often use a mixture of the two styles). For example when closing the newscaster typically says 안녕히 계십시오 instead of 안녕히 계세요. If you want to know more, wikipedia has a page on the Korean speech styles: https://en.wikipedia.org/wiki/Korean_speech_levels