The difference is in the use of the tetragramaton versus other references. Traditional translations (like JPS, but I think also many Christian ones too) translate the four-letter name of God (YHVH) as "Lord" or "LORD". Other ways of talking about God do not get the same treatment.
In Daniel, what is translated here as "O Lord" is almost always אֲדֹנָי ("Adonai"). (I see one use of אֱלֹהַי ("Elohai") in v18 translated "O Lord"; there might be others. This is literally "my God" and is translated that way by JPS in v4.)
Daniel 9:4 illustrates both uses:
וָאֶתְפַּלְלָה לַיהוָה אֱלֹהַי, וָאֶתְוַדֶּה; וָאֹמְרָה, אָנָּא אֲדֹנָי הָאֵל הַגָּדוֹל וְהַנּוֹרָא, שֹׁמֵר הַבְּרִית וְהַחֶסֶד, לְאֹהֲבָיו וּלְשֹׁמְרֵי מִצְוֹתָיו.
And I prayed unto the LORD my God, and made confession, and said: 'O Lord, the great and awful God, who keepest covenant and mercy with them that love Thee and keep Thy commandments...
In researching this answer I found that some copies of the Hebrew alter the tetragramaton in the Hebrew too. I do not know how common this is, and it may be more common online than in print editions of the text. (The concern is about printing out and then throwing away, or otherwise not treating respectfully, a page containing the divine name.)
Please note: This answer was written for a neutral, academic audience
and is not intended to be interpreted in the context of a religious
belief or doctrine.