>>935
The word "or" has many meanings, so you need to make sure what the word means in the sentence you are translating.

(1) "Or" is used to introduce another possibility.
If you are asking someone to choose something, the most commonly used Japanese equivalent for this "or" is "それとも soretomo", which is a conjunction.

As for "ミルクですか？砂糖ですか？", you can say:

"ミルク、それとも砂糖？",
"ミルク、それとも砂糖ですか？",
"ミルクですか、それとも砂糖？",
"ミルクですか？　それとも砂糖ですか？"
There is no significant difference among them.

Note: The last example shows the conjunction "それとも" can join not only two nouns but also two sentences. You can start a sentence with "それとも" that follows the other suggestion.

"映画を見に行きませんか？　それとも買い物のほうがいい？" (Why don't you go to see a movie? Or do you like shopping better?)

The postpositional particle "か" also very commonly used to make alternatives a chunk. It can work with nouns (or noun clauses) only. While "それとも" often implies that one of the things referred to should be chosen, "か" doesn't.

"コーヒー か お茶を下さい" Give me coffee or tea.
In this case, you don't care which is chosen. You just named some possible choices.

"または" is also used in written expressions. For example, when you write a recipe, you can write, "そして、それを大きな鍋（なべ） または ボールに入れます (then, place them in a large pan or bowl)". "大きな鍋 か ボール" is also okay.

(2) "Or" can mean "otherwise" when you give somebody an advice or a warning.
Japanese often say "さもないと", which is an old way of saying "そうでないと (if it isn't so)". The formar sounds more compelling than the latter.

"しっかり勉強しろ！　さもないと退学だ" (Study hard! Or you'll be expelled.)

(3) "Or" can mean "in other words." Japanese say "つまり", "すなわち" in this case.

"それは１キロ、つまり(or すなわち) ２ポンドを 少し こえる 重さがある。" (It weights a kilo, or just over two pounds.)