おか in 言っておかなきゃ

[MeitanteiJesus]

I'm curious as to the exact construction of the phrase 言っておかなきゃ

I know it means "(I) have to say it", and I know 言って means "say" while なきゃ means "unless / have to do". But I'm not quite sure what the おか in the phrase adds to it.

[NileCat]

The expression 言っておく is made of 言う and おく（置く）.
The verb 置く has a lot of meanings like the English word “put” has. It basically expresses that you “place” something like in バナナをテーブルに置く(put a banana on the table). In the expression 言っておく, the object you “place” is your “words”. The effect is that it expresses that you want to focus “the words”, or, you want to make sure that your words are placed there with no mistake. And in many cases it has a nuance that you want to state it “before you leave” or “before something is finished”.
e.g)
名前（なまえ）を言います　I’ll tell you my name / I’ll let you know my name
名前（なまえ）を言っておきます　I’ll let you know my name (before I leave / before you leave / just in case / for your reference / because I really want to)

[LordOfTheFlies]

I just wanted to chime in and add some things to nilecat's explanation.

言っておかなきゃ is a short version of 言っておかなければいけません. the ～なきゃ form is a shortened form of ～なければ and the いけません is usually omitted completely in casual speak because unless there's something following the ～なければ construction you can tell that the person wanted to say ～なければいけません. The ～なければいけません construction means that you're compelled to do something or feel the need to do something, literally translated it becomes "If I don't x, then that's bad". It's the way of saying that you have to do something in Japanese, it's a bit different from English but you'll get used to it.

So basically 言っておかなきゃ means something to the effect of "I've got to tell (someone) (something) (before I leave / before you leave / just in case / for your reference / because I really want to)".

[MeitanteiJesus]

Thanks! I'll definitely keep this in mind when I see おく in my future readings.