otousan お父さん

The word otousan means "father" in Japanese (and sometimes refers to one's husband). It's one of the many family words with the o__san お〇〇さん pattern, and as such the honorific suffix can be changed between san, chan and sama.

Kanji

The word otousan written with kanji is otousan お父さん. It has the same kanji as another word for "father," chichi 父, but uses a different reading (and has different usage). The other words with different suffixes are written similarly:

otouchan お父ちゃん
otousama お父さま

The sama 様 suffix and the o 御 prefix are only sometimes written with kanji. So, for example, otousama can also be written お父様 or 御父様.

Otouchan

The word otouchan means the same thing as otousan, "father."

The difference between otousan and otouchan お父ちゃん is that otouchan is a cozier word. It can imply a more friendly, relaxed, or intimate relationship between the speaker and the otouchan.

Otousama

The word otousama means the same thing as otousan, "father."

The difference between otousan and otousama お父さま is that otousama has more reverence. In anime, this often implies that the character is from a traditional family (often rich $$$) which believes children must address their parents, grandparents, etc. with respect.

In some cases, it can imply the speaker has admiration or esteem toward their father.

Tousan, Touchan, Tousama

Sometimes the word is pronounced without the o prefix. This is just a more informal way of saying it.

tousan 父さん
touchan 父ちゃん
tousama 父さま

Otousan vs. Chichi

The difference between otousan and chichi is that otousan has honorifics. Normally you don't use honorifics toward yourself, so you say chichi when you refer to your own father, and otousan when you refer to someone else's father. One exception is when you're talking about your father to someone inside your own family, then you use otousan. (see お父さん vs. 父)

Some example phrases:

watashi no chichi¹ to anata no otousan² 私の父とあなたのお父さん
My father¹ and your father².
anata no otousan wa genki desuka? あなたのお父さんは元気ですか？
Is your father well?
okaasan, otousan wa? お母さん、お父さんは？
Mom, [what about] dad?
Mom, [where is] dad?
(here the speaker is speaking to their "mom," asking about their "dad.")

Usage Toward Husband

In some cases, otousan may refer not to one's father but to one's husband. This mainly happens when a mother is speaking to their child about the child's father, who's their husband. So a phrase like "otousan will do this" may mean "the father in this family, who's my husband, will do this."

There are couples who call each other like that even when not in the presence of their children. On the other hand, there are also people who think doing that is weird.

Otousan vs. Otōsan

The difference between otousan and otōsan, with a macron, is merely that they're different romaji for the same Japanese word. The same applies for otōchan and otōsama.

This happens because long vowels, found in syllables like too とお, tou とう, and too とー, may be romanized with a macron instead of two vowel letters.

おとーさん

Sometimes when otousan is written without kanji it becomes otōsan おとーさん instead, with a prolonged sound mark ー. This often happens with child characters to denote they way they speak. Likewise: otōchan おとーちゃん, otōsama おとーさま.