okaasan お母さん

The word okaasan お母さん means "mother" in Japanese (and sometimes refers to one's wife). 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 okaasan written with kanji is okaasan お母さん. It has the same kanji as another word for "mother," haha 母, but uses a different reading (and has different usage). The other words with different suffixes are written similarly:

okaachan
お母ちゃん
okaasama
お母さま

The sama 様 suffix and the o 御 prefix are only sometimes written with kanji. So, for example, okaasama can also be written お母様 or 御母様.

Okaachan

The word okaachan means the same thing as okaasan, "mother."

The difference between okaasan and okaachan お母ちゃん is that okaachan is a cozier word. It can imply a more friendly, relaxed, or intimate relationship between the speaker and the okaachan.

Otousama

The word okaasama means the same thing as okaasan, "mother."

The difference between okaasan and okaasama お母さま is that okaasama 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 mother.

Kaasan, Kaachan, Kaasama

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

kaasan
母さん
kaachan
母ちゃん
kaasama
母さま

Okaasan vs. Haha

The difference between okaasan and haha is that okaasan has honorifics. Generally speaking, you don't use honorifics toward yourself. So you say haha when you refer to your own mother, and okaasan when you refer to someone else's mother. One exception is when you're talking about your mother to someone inside your own family, then you use okaasan.

Usage Toward Wife

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

Okaasan vs. Okāsan

The difference between okaasan and okāsan, with a macron, is merely that they're different romaji for the same Japanese word. The same applies for okāchan and okāsama.

This happens because long vowels, found in syllables like aa ああ and aa あー, may be romanized with a macron instead of two vowel letters.

おかーさん

Sometimes when okaasan is written without kanji it becomes okāsan おかーさん instead, with a prolonged sound mark ー. This often happens with child characters to denote they way they speak. Likewise: okāchan おかーちゃん, okāsama おかーさま.