Meaning of “Otaku(オタク / ヲタク)”, “Ota(オタ/ヲタ)”

“Otaku(オタク / ヲタク)” means nerd or geek or a enthusiastic fan worshipping of something.
Both of オタク and ヲタク has same meaning, but in my opinion, ヲタク is more Otaku than オタク.
Basically “ヲ” is used as a postpositional particle, so オタク is formal word but ヲタク is a kind of slangs.

If someone say just “Otaku”, it means nerd or geek.

If someone say [something] otaku, it means a enthusiastic fan of something.
In this case, you can also say [something] ota(オタ/ヲタ).
Ota(オタ/ヲタ) is an abbreviation for Otaku(オタク / ヲタク).
And you should shorten [something] to around two characters, if possible.
(Sometimes it’s hard to shorten it, then it’s OK if you can’t shorten it.)
Japanese like to shorten words to three or four characters, so there are already two word “ota(オタ/ヲタ)”, so the something should be around two word.

There’s a wide variety of “Ota(オタ/ヲタ)” in Japan.

AniOta(アニオタ)
“AniOta(アニオタ)” means a enthusiastic fan of animation.
“Ani(アニ)” is an abbreviation for “Animation(アニメーション)”.

TetsuOta(鉄オタ)
“TetsuOta(鉄オタ)” means a enthusiastic fan of trains.
“Tetsu(鉄)” is an abbreviation for “Tetsudou(鉄道)” which means trains.

DolOta(ドルオタ)
“DolOta(ドルオタ)” means a enthusiastic fan of Idols.
“Dol(ドル)” is an abbreviation for “Idol(アイドル)” .

JaniOta(ジャニオタ)
“JaniOta(ジャニオタ)” means a enthusiastic fan of Johnny’s idols.
Johnny’s is the most famous talent agency of boy’s idols.
(This is the Johnny’s web site.)

Example Conversation

A: I found out that my girlfriend is a JaniOta…
B: Oh, so that’s why she is always short of money.
A: She spends a lot for her Oshi…
B: Oh…

A: 俺の彼女、ジャニオタだって発覚した…。
B: え、だからいつも金欠だったの？
A: 推しにお布施しまくってるらしい…。
B: おおう…。

※In this case, Ofuse(お布施) means pay a lot of money for oshi(推し)