If you have not been on orals recently:
-Increase phosphocreatine storage, which will immediately/temporarily cause an increase in strength. When you get off, you will lose some of this. This is something very unique about orals, and one of the reasons they are so powerful for strength increase.
-CNS will be more "revved" up , as you probably know. Once again, orals are very good at this. Even though Oxandrolone has a low androgenic ratio, it is very good at increasing strength. People really underestimate it's ability to add strength (or they get it fake, and think Ox sucks when it doesn't...their product was just not really Ox).
-Expect muscle hardness/fullness. This is even more noticeable as you increase the dosage to 75-100mg(which I am not recommending,but just stating facts). It is obviously also more noticeable if you are leaner and have less bloat.
-Some extra muscle gain
-Less water retention
-More prevalence of noticeable veins
-It really "kicks in" for people somewhere between 4-10 weeks. My hypothesis is that far too many people get fake, or blended, or underdosed Ox. Although people vary in their reaction to medications, there is far too much anecdotal evidence showing significantly different figures in regards to this.
I think if you have real Ox, and it is properly dosed, you should feel 50-75mg really kicking full circle even by week3-4; there is obviously chance of variance.
Concerns:
You are already overweight. 330 pounds, regardless of height or how much of it is muscle, is very taxing on the body(this is a true,nonbiased fact). Your cardiovascular system and internal organs are stressed as it is, regardless of whether you have or don't have evidence to support that notion IN THIS VERY MOMENT.
Adding an oral will be even more taxing on your system since it will come with the slew of side effects you most likely already know about:
LDL increase
HDL decrease
Liver function decline
Cardiovascular stress
Kidney stress
Obviously the LDL/HDL/Total cholesterol figures will most likely be the primary concern in this scenario.
People will claim that oxandrolone is a very 'mild' steroid and some even have the nerve to call it "baby food". This is naivety at it's finest, and in my personal opinion: the mindset of somebody who abuses things without taking into consideration the possible implications of their actions.
Oxandrolone can, and most likely will tank your cholesterol levels. People will argue that this "temporary" effect (which could last for 1+ month even after being off depending on who you are), does not have clinical evidence of causing issues. Oh really? How many clinical trials have we had of powerlifters that weigh 300+ pounds taking steroids and following their HmG-CoA reductase and increase the risk of cardiovascular disease/stroke/myocardial infarction? Lol. You will see terror stories all over the internet about Anadrol being so hepatotoxic and terrible for lipids. I can guarantee you, that this belief that compounds like Anadrol/Dianabol are much harsher than Oxandrolone are essentially false. In fact,I would go on further to say that for the vast majority of individuals would have better bloodwork and less system stress on 50mg of Anadrol a day versus 50mg of Oxandrolone. But that's not the common belief around the internet now...is it?
Look -- the reality is you're probably going to be fine. In fact, I can almost guarantee you 95% that you will be okay running this cycle, especially if you run the minimal of 40-50mg a day(which is what I would think you would run at a minimum considering your experience). However, there is ALWAYS a possibility something to occur. So please be very very careful with all orals, including this one

, I want you to stay healthy!
I think the situation would be very different if you were only 200-220 pounds, for example, and taking 40-50mg of anavar a day. But at these heavy weights you're carrying around, I am very concerned because it can be dangerous adding anything to your regimen, including poor food choices. I'm really looking out for you.
If you feel like the benefits outweigh the possibilities of complications, and you feel that you will be responsible enough with your training/supplementation/blood test analyses, then go ahead and let everyone know what you plan to run.
Since it's your first time, perhaps you would like to just try 50mg for 6 weeks? I'm being honest here, and I have a feeling you won't really "appreciate" the drug unless you take around 75mg for 6-8 weeks. You may also be one of those individuals that does better on 4on/4off with orals; but I don't like that idea *personally*.
PS: I have serious powerlifter friends putting up extreme numbers(800+ raw on lifts), and I am 100% aware of what they are using. I already know you might be looking at me after what I wrote and think I possibly am clueless to the reality of things. I truly am not. To give you an example, one of my good colleagues runs 200+ anadrol a day, alongside 20 superdrol...and those are just his orals...and one of them is an extremely harsh PH compound. So yeah, when looking at the subculture of powerlifting around you, perhaps it doesn't seem like a "big deal" to take 50+mgs of anavar a day. But be real with yourself and recognize this sport is not healthy whatsoever, and if you are 330 pounds, there is a chance that anything could offset some kind of system distress(especially poor diet,orals,tren use,etc).
Good luck =)