In OREMO there are three recording modes, accessible via Option > Recording Style Settings.
Manual: Press and hold R to record. If you're getting cut off at the ends of samples, please double check that you are holding R long enough.
Automatic 1: Press R, and it will play guideBGM once and automatically record. The default one (F, 100bpm) has long enough recording time to handle 7 mora, but if you want to use something like an 8 mora reclist (Which I recommend if you are able to maintain your voice and not lose breath) you'll have to get a different BGM. One Note Jazz is popular, and you can download it here.
https://drive.google.com/open?id=0B43rIhv9uaxIbV92TnFVRWw5OHc This mode is good for if you're new to using OREMO for VCV.
Automatic 2: Press R, and it will play guideBGM repeatedly. It will automatically go to the next sample in the list. This mode is excellent if you know your reclist well and would like to be as efficient as possible.
OREMO comes with a reclist named reclist-renzoku-NHP.txt that stands alone well. If you want a more efficient reclist, you can extract Yokune Ruko's 8mora. I write my own reclists, and if you'd like a highly efficient reclist that has only the essentials of japanese, you can PM me for it. Other people have written efficient reclists as well. This one has shorter strings and also comes in romaji, all with a base OTO.
http://utaforum.net/resources/no-nonsense-japanese-vcv.192/
If you record with a hiragana VCV reclist, and with OREMO's guideBGM, you can use the built-in VCV oto generator to highly accurate results.