Here is a little bit about the process. (and it seems you have already done the steps) but I'll share regardless

Write a letter saying that you suspect the disability is having an adverse educational impact on your child and that you are requesting an evaluation for an IEP that would provide special instruction to your child.

MCPS must schedule an "IEP screening meeting" within 30 days at which time they will consider whether there is "reasonable suspicion" of an "educational disability" that necessitates "special instruction". At this meeting, you must make an argument with some supporting evidence on the three prongs of the test for an IEP -- 1) disorder, 2) adverse educational impact and 3) need for special instruction. ADHD qualifies as one of the disorders under the "Other Health Impairment" category of the main educational disability law, IDEA.

If you pass the "screening meeting," the school system will be obliged to evaluate your child. At this screening meeting, if you pass, the team will decide about evaluations. This is the time to specifically request the OT evaluation (not really clear why you want this as part of the ADHD evaluation, but make the argument why and ask.) IME, the school psychologist asks you to sign a pretty blanket consent to evaluate. If you want something specific in terms of evaluation, request it in writing. The school psych will, at a minimum, do an IQ tests, some achievement testing and a classroom observation. Parents and teachers will be asked to fill out a checklist.

MCPS has 60 days in which to conduct the evaluation and schedule the final "IEP determination" meeting. At this meeting, the three prongs of the IEP test will be re-examined, and if you can prove all three prongs, then school will have to provide an IEP. There is then another 30 days to write the IEP plan.

The meeting isn't to screen for a disability- it is to determine what tests are needed to determine needs. Even with a diagnosis, the school team might need additional tests to determine what educational impact there is. The results of the tests will be used to prepare a plan for you child, so go in with a positive attitude and assume that they have your child's best interests in mind as they suggest different tests. Now, if that does not seem to be the case, get an advocate to help.

That makes sense. I guess it seems odd to me to have a screening meeting to assess for possible disability when he already has a diagnosed disability that they are aware of. It seems like a needless step. It seems like the whole process with all three steps is going to take most of the year. Sigh.

In MCPS there are a series of 3 IEP meetings:

1. The screening meeting to determine if a disability is suspected based on the educational history, an observation, teacher reports, report cards, parent report, any testing etc. At this meeting, testing would be recommended or the IEP process can be stopped.

2. The eligibility meeting where the assessments are reviewed and either eligibility for special education is determined or you not not determined.

3. If found eligible for Special Education services, you would have your 3rd meeting, which would be drafting the IEP.

I hope this makes sense. If you are confused. Hire a consultant.

I know this can be a frustrating and confusing process, but try going in to the meeting as calmly and professionally as you can.

In general, a screening meeting is needed to determine exactly what types of tests and data they need to determine eligibility. Kids can have IEPs for all sorts of issues, and a kid with communication issues will need to be evaluated using different tools/tests than a kid with behavioral issues vs. a kid with gross motor needs. So the first step is to get the group together, look at where the kid is, hear concerns from parents, teachers, and anyone else that's relevant, and figure out what testing would be appropriate. It's not intended to delay or be mere bureaucratic nonsense (although I'm sure in some cases it is!)

To prepare, go in with a very specific list of your concerns and how they are impacting your child's education. Remember to focus on the educational impact.
I don't think you need an advocate for this meeting, but if you're rolling in money and it will make you feel better, you can certainly bring him. I do like to make sure I have my spouse, though, so that there are two people giving their perspective and really hearing what the school staff is saying.