Now it's time to present it to the world, and defend it to your academic colleagues! This is exciting, and also stressful. That's normal. Do what you can to prepare (give a practice talk at least several days in advance to some supportive friends and incorporate their feedback for improvements; run through the talk at least several times until you are confident you have the timing down). After that, don't worry about being nervous. My experience with giving talks is that I am always nervous, and that's ok; I can be nervous and still do a good job.
Here's my advice on structuring your presentation.
For the session with your opponent, be prepared for both big picture and detail questions, on both your written thesis and oral presentation. The following are by no means exhaustive, but just a sample of some kinds of questions that have been asked in my experience (of course your own experience may vary).
It's OK to take a moment to consider your answer, or to ask for clarification about what the opponent is looking for.
Here are some questions you can prepare to answer (out loud on your own, or at the end of your practice talk with friends):
- Why did you choose this study system?
- OK, you work on carbon sequestration in grasslands. What % of the global carbon budget are grasslands? Do they sequester more or less than forests? Similar question from a friend’s defense: why is it that the Ross Sea is so productive compared with other global oceans? (Put your work in larger context.)
- What is the contribution of your dissertation? [to X field]?
- How do you know that XYZ assumption you made was appropriate? Why was this method (regression analysis, interviews, etc.) appropriate for the question you wanted to answer?
- How would you demonstrate X? (Might be a logical next step/extension of the work you did, or might be a totally off the wall thing that makes no sense, partly testing you on whether you recognize this.)
- What is the mechanism that drives the result you found? Does it apply in other systems? How?
- What does the literature say about your findings/this work? Who are the people at the cutting edge of this field? Who are the people who would disagree with your findings? Related: So-and-so just published a Science paper/gave a talk on X (may be closely or not so closely related). How does this relate to your work? Do your findings agree or disagree with theirs?
- You should have measured/done XYZ instead. (This is the kind of thing that reviewers say all the time so I think this is to prepare you for that. Talk about how that would be interesting to answer a different question/with more resources, but for your question, your approach was solid.)
- Explain the two-year reproductive cycle of the grapevine, draw the global patterns of nitrogen availability, temperature & C3 vs. C4 grasses (... various specific things with right and wrong answers... hard to "study" for these as they could be about anything even tangentially related... perhaps spend 30 minutes glancing over a grad-level textbook most closely related to your field to jog your memory, but most importantly stay calm, use logic, draw on the vast amount of trivia you’ve been filling your brain with for the last 5-10 years, but admit when you don't know an answer and say where you would go (or what you would do experimentally) to find it out.)
- What are your next steps? (Both in terms of this research, and professionally, including where do you want to be in 5-10 years.)
- What are the management or policy implications of this work? How have/will you communicate these to managers/policymakers?
- What are the theoretical implications/contributions of this work? (Put in context of broad concepts of the field, not just literature but bigger picture, throw some terms around that relate to theory like “vulnerability analysis,” “mass balance,” whatever).
Good luck! You'll do great!