Just have a couple of questions about the EPI:
1. In the results section, what can I comment on without doing too much explanation?
2. Where can I justify the use of oxygen gas production as a suitable dependent variable?
3. How can I comment on whether the investigation generated useful results?
4. How can I effectively comment on the meaning/significance of results obtained?
Thanks.
(1) Firstly, within the results section, comments or any descriptions are often unnecessary, merely data points and charts are adequate. I must mention, however, if your results have a qualitative component then laconic descriptions such as 'solutions altered from pink to blue' are sufficient etc.
(2) In terms of oxygen gas production acting as a dependent variable, justification throughout your poster is often implicit, that is, one doesn't often need to explicitly state that it is an appropriate dependent variable because... Rather, focus on identifying the process being observed in your introduction (i.e. photosynthesis), then briefly outline how it works (here you can acknowledge that oxygen is a product of the light-dependent reactions).
(3) In my opinion, this can be done in the discussion (in a far more detailed manner) or in the conclusion where it can be hastily stated and supported. (i.e. the findings of this investigation, in coinciding with current scientific theory, acts to bolster what is currently known, thus demonstrating the credibility of [insert aim of investigative research here etc].
(4) Lastly, when interpreting results (i.e. significance of results), this can be performed within the discussion where further drawings upon the data can be established and used to explain other scientific concepts etc.
Good luck

DISCLAIMER: I recommend you discuss these questions with your teacher when possible. My suggestions are merely my own opinion and due to the subjectivity of various schools' marking schemes for the Extended Practical Investigation, your teacher/(s) may expect different renditions of the poster to what I envisage.