I am by far no expert as I have just started on my venture of OAMS, however I feel like as a beginner I can share what few things I've learned that have helped me. Be forewarned this is a long post but hopefully you will find it helpful if you are getting ready to try OAMS.
The first thing I did when I started thinking about trying OAMS was google search it. In doing so I found dozens of other blogs with statements of how they organize their trips, make their lists, menu plans, budgets, etc. I read every one I could find to get the concept and tried to figure out what would work for me and my family. As you will quickly find out, what works for one person fabulously may not work for you and your family in the least. Most OAMS shoppers have deep freezers to hold their massive quantities of food as well as large pantries. When I realized this trend while reading multiple blogs I started to get discouraged thinking because we lived in a 2 bedroom apartment with no deep freeze and limited cabinet space I wouldn't be successful. This is not true. You may not be able to stock up on a months worth of milk and bread without a deep freeze but you can certainly stock up on many other things with cabinet organization and thinking outside the box in terms of storing items.
I also read many people were doing the bulk of their shopping at Costco and Sam's. I have never shopped at either so I went online and started to compare prices. Turns out at this point they couldn't beat what I could find with sale prices and at my local Walmart. So I started by making a list of items we use often and could buy in bulk, olive oil, veggie oil, flour, sugar, toilet paper, gallons of juice, oatmeal, brown sugar, powdered sugar, etc. I then went to Walmart, Meijer, and Aldi and wrote down the prices and sizes of each of these items so I could find out where the best prices per size were and so I would know if a sale was actually a great price. I found that buying a 25lb bag of flour or sugar at Walmart is not necessarily cheaper than buying individual 4 and 5lb bags at other stores, in which case the "in bulk" buying is pointless as I find it a lot easier to find storage for 4 individual 5lb bags of sugar than 1-20lb bag. I keep my price lists in a notebook along with any other information I come across that might be useful, but I only use this notebook for OAMS info.
Buying the larger package is not always a savings. My husband and I were recently at Walmart to get non-grocery related items when he informed me we were down to the last slice of cheese (he, of course didn't inform me of this before my OAMS trip and when I did my inventory days earlier we still had plenty). So we went to the grocery section and he started to pick up the 24 slice package for $2.98. I had bought this size just a week earlier and told him I had no desire to make another trip to the store in a week for more cheese so I picked up the large 72 slice package for $10.98. When we started to make our way to the checkout I started doing the math: 3 packages of 24 slice cheese (72 total slices, the same amount in the large package I was planning on buying) totaled$ 8.94, and yet I was getting ready to pay $10.98 for the same amount of cheese. It was the same brand and everything so I of course put back the big package and bought 3 of the smaller ones. So always make sure to figure up if your actually getting a deal by purchasing items in bulk.
I also started putting dates marked on masking tape on just about everything when I open something. This way I know how long it takes us to run out of that item. I keep this written down in my OAMS notebook as well. The point behind this is so I won't run out of things mid-month and have to do a larger grocery trip than planned. I will know we go through a bag of sugar in a max of 10 days, so if my main monthly shopping trip is planned for this weekend and I know we have 2 bags of sugar left in the pantry, I know I will have to buy at least 1 more to have enough for the entire month. Waiting to run out of items before buying them when you're trying to do OAMS defeats the purpose of making a huge shopping trip at the beginning of the month.
I didn't actually have a "budget" for my first shopping trip as I really wasn't for sure how much we had been spending. I knew how much I had been intending to spend, $120/month, but we were spending that entire amount within the first 2-3 weeks. I was also using my own monthly "spending money" for small grocery purchases at the Dollar Tree and quick trips to the store which I wasn't counting in my monthly grocery spending. This unknown is why I chose to pick a month we had a little more wiggle room with finances to start my OAMS. We had run out of many of the basics and were very low on canned veggies, baking items, and meat, so I knew it would cost more than I would plan to spend. I plan on keeping a detailed record of how much we spend throughout the month including our weekly small trips so I will have a more accurate view of what our budget should be.
Before I even made my first trip to the store I made a list of every dinner, breakfast, and snack item I had ever made. I saved this list to my computer for a quick reference when making out my monthly meal plans. It will save you loads of time and it keeps me from repeating the same meals because my brain farts constantly and I can't think of anything to cook. So when making my monthly menu I go over the list and start choosing what I think we should eat that month. I also write down a few snacks and breakfasts so I don't forget to write down the ingredients on my grocery list and so I don't forget to make them. I get very caught up in making sure I have everything I need to make multiple dinners that I forget Grant also needs snacks during the day and a home cooked breakfast a few times a week. So it's a necessity to have these things written down on my meal plan so I know what I have the ingredients to make.
Every month when you are sitting down to make out your menu plan, look back at your menu from the previous month and enter in any meals you ended up not cooking for whatever reason. Because of holidays and unplanned things I ended up not cooking at least a weeks worth of meals I had planned for November. I immediately write these meals in for the month of December because I already know I have all the ingredients to make them. This saves me on my grocery budget for the trip I'm planning and allows more wiggle room to buy bulk items or more meat.
If possible buy more than you need. If your budget is super small, and sometimes depending on what we need and what unexpected events have occurred, ours is, then you may not be able to buy anymore than what you absolutely will use. However even if you have an extra $5 you can buy extra of something. I try to buy large bottles of veggie oil and olive oil even though I could buy a smaller, less expensive bottle to get through the month. I also buy extra bags of flour, sugar, and brown sugar, along with extra meat than we will typically use. I always buy the family package of meat and separate it into individual or meal portions. In November I bought a family package of Chicken consisting of 7 boneless skinless chicken breast. When I got home I did as I always do and cut them in half making 2 thinner chicken breast (which is more than enough for one person if we are eating marinated/grilled chicken). So I ended up with 14 chicken breasts from the one package. There is no way we can use that many in one month so they actually carried over into December and part of January. This saved me from having to buy that meat and gave me extra meals to make without adding to my grocery list.
If you are planning on going to multiple stores to get all your months worth of groceries make sure you go to all those stores in the same day. If you don't it will inevitably happen that something will come up or you won't feel like going to the store you put off and you will be lacking an ingredient you need for a meal that month. There is nothing more annoying with OAMS than spending time making your list and meal plans only to get toward the middle of the month, get ready to make a meal, and realize your missing one ingredient. I did this my first month (November) and swore not to make the same mistake again.
My biggest tip when starting your OAMS venture is to get real with yourself. By this I mean, be completely honest about what you use, how much of it you use, and how quickly you use it. Over the last couple months I have found myself doing what I used to do when I was shopping weekly. I would think to myself, for example "we have half a bottle of veggie oil, I should put that on my list but my budget is almost maxed out and we can probably get through the month with 1/2 a bottle". I used to put off buying things for the sake of staying in budget all the time with my weekly trips. This is how you end up running out of all your basics and you eventually are forced to go way over budget in order to have basic ingredients to cook your meals. It is better to possibly go over budget by a few dollars one month than to run out of multiple items and have to go over by $20+ another month. I now keep a list on my fridge and whenever I notice we are running low on any food, spices, cleaning products, or toiletries, I write it on the list. This will save you from having to do a large pantry inventory every month before your trip.